Well, Mr. Retl, mostly it's because I enjoy doing it a lot. It's a time that I can just throw on some relaxing music and think up events and characters that nobody else has. For a lot of people, they can get lost in things like games, a TV series, or even with reading. But for me, hours melt away for just a few thousand words. And after I'm finished, I can step back and look at it, then either say I like it, or not. And if not, then I just get to put my music back on and do it all over again. Writing for me is an escape, and so I like to do it as often as possible. Which, I know isn't the case with everyone, but that's where I'm coming from.
The main thing I can suggest for you though, is that you have to like what you're writing. Like, really truly enjoy what you're writing. Even if it's something that only you alone can look back on and say you like it, you're still the only one that has to feel good about it. If it's not fun to write, or you're getting frustrated because nothing's coming out that you like, take a break. Or alternatively, don't, and simply redouble your efforts.
There really isn't a wrong answer to how often to keep up on writing, if you should take long breaks or short. I've had days and even weeks set aside away from writing because of outside plans, or things like depression soaking in. But that's okay, because so long as you're enjoying it when you do work on it, that's all that matters. And in the end you do really enjoy writing despite having difficulty getting anything down, then just keep trying, even if you have to take breaks from it.
And if the case is that you look back on your work and question how good it is, that's pretty much normal. Again, only you have to enjoy your work, and know that the more you end up doing it, the better you get. The downside to that is that it makes looking back even worse and harder to accept. That's where a lot of people decide that it isn't worth continuing, when the truth is it's the perfect time to keep moving forward. Know that you'll make mistakes, that you'll have gaps and errors in the story. Push to the finish, take what you've learned, and then do it better right from the start of the next story.
Anyway, I know I tend to kinda ramble on, but that's pretty much it. Enjoy what you are trying to write, and don't stop to look back until it's finished. It's not a sprint, it's a marathon that you can and will finish if you pace yourself to where you feel comfortable. The only person to put pressure on you when you write should be yourself, and that's only so you can give it your best right up to when the last sentence of the last paragraph is finished. :3