I do tend to have similar motivation problems (motivation to start, continue and finish things), although most of the time it's not related to drawing itself but to the idea.
For the problem itself (loosing motivation because it doesn't turn out right), to be honest, the only thing I can tell you is: just go on and finish it. If you never finish something, you will never learn how to do it. I know it is not easy, I have that problem all the time myself, and to be honest sometimes I just want to give up when I see the terrible results. This is were the motivational stuff kicks in; so here is a Top 5 of what works best for me:
1) Listen to music.
That almost always works for a short but strong boost of motivation. The best is actually to have some kind of music you somehow link to what you're drawing (e.g. when you draw a character being badass, I listen to something like this
If whatever you're working on is too general or you can't relate it to anything for any reason, you can try songs that make you think about work and why what you are doing is good. (again, here's what works for me: link
2) Draw something easier to relax.
This works quite well since it makes you regain some confidence in your work as you have no problem doing it exactly the way you want. Note however that this is a hippopotamus-sized time killer, but I guess it is still better than doing nothing at all.
3) Think about why you want to draw this.
Now this one is a bit more tricky and situational, but it helped me already out of the worst cases of depression. Usually, when you "fall", you start to doubt the use of your work, both in terms of meaning and the advantage you gain out of it. Thinking about precisely this will call back up the roots of your original motivation to work on your drawing and make you "believe" again. A main part of this is telling yourself that unlike all those nice drawings made by other people you see out there, this is and will be the results of nothing but your effort.
As is said, this is very hard to pull off. You could compare it to starting a very old car: it might take several turns of the key to get the engine running again.
4) Look the solution up. (If the thing not turning out right is practical)
If for example you don't manage to draw a hand, look some hands up on google, search some tutorials and how-to's on DA or in any case try to find a model for what you're trying to do. Pair this with practising the thing several times on a separate piece of paper till you get it right without that model. Quite time-expensive too, like 2), but that's the best way to go for that kind of situations.
5) Ask someone.
That's why you're on this forum. Try to finish you're sketch even if it isn't like what you expected(If you really are exasperated, just "fill up" void spaces with simple stuff), show it to someone and point out the problems you have.
Loosing motivation for this kind is related to not being able to find the solution of the problem. And other people might already know about it or may have a different point of view on the subject which allows them to find what is wrong and how to fix it. Plus, knowing that there is someone who cares about your problem and supports you in your work is always a good thing.
Well, that is all I can think of. I hope it helped you.
If not, bash me wish a frying pan.