Whenever I'm a little depressed with my sales (and of course I usually have only myself to blame for it), I try to cheer myself up by figuring out where I am in relation to long-term goals. Basically, my dream is to earn XXX amount each month. This amount is sufficient to pay for household expenses and still have enough to set aside for savings as well as for my travel fund (my #1 weakness). Oh, and this amount also includes what I pay for taxes and advertising expenses for my books.
To arrive at this amount, I use the calculations that I borrowed from Joe_Nobody of Kindleboards. I already discussed this before I think, but in case you don't want to search for it in my archives then here it is again.
(COMPUTATION BELOW IS FOR SAMPLE PURPOSES ONLY)
Monthly Income Target: P90,000 (this should leave you with about P50k every month once taxes and advertising expenses are deducted)
Estimate royalty per $2.99 (my regular price for ebooks): P86
Average number of copies sold per book per day: 5 across all retailers (let's aim for a really small number, meaning even if you don't lift a finger, you will still surely sell this number per book each day)
Royalty per book per day (P86 x 5): P430
Daily Income Target (P90,000 / 30 days): P3,000
# of books you need to publish in your lifetime (P3000 / P430): 6.97 or round up to 7 books.
To be safe, let's just say you need 8 books in total. Eight books priced at $2.99, selling FIVE COPIES A DAY EACH. If you can manage that, you can then earn a gross monthly income of P90,000 per month.

Right now, I am at 19.8% of my target number of books to be published in this lifetime. Knowing this number makes me excited and it definitely gives me motivation to write. One reason why I want to reach my target quickly because when I do, I will have more time not just to spend with my family and loved ones but it also means I'll have more time to write in advance. I can write an entire series and then release it all at once, which I know is how readers usually prefer to read books in series (I'm kinda the same as well hehe). 

So, yes, I was a little depressed earlier, but now I'm not. :D Off to writing so I can be another book closer to my target. Wee!!!

Finishing a book - or, technically speaking, a story / manuscript - is always the hardest the first time around. And yes, I definitely speak from experience. After the first book, it's possible to still find the second and even third or fourth books equally hard to finish. Past that, there may be still difficulties - but in my experience - they definitely aren't as insurmountable as the ones you had to tackle with your first book.

Why is it so hard to finish a book the first time around?

• Since you don't have any previous experience to compare to, The End (the last page) will feel inconceivably far to you. It's like no matter how many words you write, you just can't quite see yourself finishing the story.
• Insecurities abound, and so you constantly rewrite.
• You want your first work to be perfect (which means you haven't yet realized this is impossible) and so you constantly rewrite.
• Unless you've been commissioned to write or your manuscript was accepted on proposal (meaning, you scored a publishing deal based on the first three chapters and synopsis of your work), you don't have a deadline to meet. As such, you can rationalize all you want about why you can't write right now. Or tomorrow. Or next week. 
• You may be genuinely passionate about writing, but you still haven't found the right motivation to finish your book. You may be writing every day, but it doesn't always have something to do with your unfinished book. 
• You're busy with school or work. Again, it doesn't mean you're not passionate about writing. You are. You just happen to be busy as well, and so you can't "afford" to devote your time to writing.

There are likely to be other reasons but the ones I've mentioned above are the most common, IMO.  If any of them applies to you, well, good news - it's normal, and you have it in you to overcome them.

What I did to finish my first book
Like many writers, I struggled to finish my book in a timely manner. And I probably wouldn't have if not for the following:

• I decided to write a detailed outline. This way, I won't waste my time writing aimlessly, which of course may mean wasting readers' times as well because I could end up writing fluff.
• I gave myself a deadline. I realized that it's impossible for me not to finish it without a deadline.
• I found the right motivation. In my case, I found local chick lit books written in English. I never even thought such a thing existed until I saw the ones published by Summit and PSICOM. It made me think I could write the same thing as well, and so I did. I wanted to be one of the first to finish and submit my manuscript. I didn't want to risk finishing the book only when it was already too late, and the market's saturated with local chick lit novels.
  

Why I still find it hard to finish a book

• Time management. It's one of my greatest faults. 
• Priorities. Sometimes, writing really does have to take a backseat for me. 
• Insecurities. There are times when I worry too much about what readers or critics would say, which then makes me question my plot, my writing - basically anything that has to do with my story.
• I don't "feel" writing a particular story.
• Another story has captured my mind, and it's what I want to write. 

What I do now to finish my books

• Outline. Process and type can change, but I can't recall the last time I wrote a book without any outline. Mm...I don't think I ever did completely. Maybe for a few chapters, but in the end, I always find myself outlining.
• Deadlines. Yes, this still works. Sometimes, I also add a condition to it. For instance, if I don't finish my book by (Date), I will (insert unpleasant stuff here). Or it could be the other way around: if I finish my book by X number of days, I will reward myself with (insert pleasant stuff here).
• I switch manuscripts. 
• I seek for additional inspiration, which usually means reading new books, listening to music, or watching movies. 
• I remind myself of how much I still have to earn by the end of the month - and how many days I have left before the month ends.
• I remind myself of the people depending on me.
• I remind myself not to pay any attention to haters. Whatever they say doesn't matter because they're not my target market.

Additional advice for finishing your manuscript (basically a combination of what I did or should have done at the start)

• Eliminate negativity in your life. If you start thinking it's impossible to finish your book, shove the thought away. If someone comes to you and say you won't finish your book, stay far, far away from that person. Negativity takes many forms, but you have to be vigilant and persevering. No matter what happens, don't let the bad stuff get to you. Just keep writing!
• Write every day. Make time for it. Force yourself to finish at least one sentence, and more often than not, you'll find yourself writing more. Once you've found your groove, you'll even end up writing more than you ever did.
• Be inspired - and keep in mind there are no fast and hard rules for this. 
• Stuck in your plot? Look for books or movies with similar twists and see what you can do to tweak it and then use it for your own story. 
• Give yourself a deadline. Have a goal. And hold yourself accountable.
• Keep writing - but avoid editing or revising. You can do the latter once you're done with the first draft. The important thing to remember is that YOU NEED TO FINISH YOUR FIRST DRAFT. You can revise to your heart's content after that. 
• Ask for advice from mentors or authority figures.
• Read up on tropes or common romance novel plots.
• Remind yourself of what you stand to gain or lose if you do or don't finish your book.