I never lose interest, I just... how do I put it... get disappointed in what I've created?
I'm mostly composing music though, and this is how it usually goes. I sit for a few minutes in front of piano, coming up, say, with a funky bass line. Once I've stumbled upon something and recorded a raw line, I add drums and spend some time looking for a decent drum beat. I keep adding more and more instruments, and eventually it sounds awesome! Then I leave it for tomorrow, tomorrow I turn on my laptop and listen to the track which I liked yesterday, and... it just doesn't click with me! I sigh and remove it.
I think I've only really completed 3 songs among dozens I've tried, removing 2 in the end. 1 is still there somewhere, and that one actually was quite decent - but I've just been too lazy to come back and polish it.
I was that way with my art for a long time. One of the reasons I started posting stuff online is because I thought it would make me feel more, "accountable" I guess. Like, if I have to own up to it, then it has to be up to a standard of quality I can accept. And it really did help. I could tell that I was making improvements with every drawing.
Really though, there's no point in feeling disappointed in your creations, if you're making an effort to improve. Even if in the end it's not the greatest thing ever produced, you can point at it proudly and say "I learned from this".
Anyone want to read a poem I wrote 13 years ago?