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You've only just started making games, right? You've already done some really cool stuff, and you still have the rest of your life to make better stuff :D And like unity said, if you're having fun during the process, then slipping up here and there is no big deal.

I don't think that feeling of "I could be making something better" ever really goes away, even if you've been making games forever! I still get that way all the time, at least, and I see people who are much more talented than I am say similar things. But, y'know, if you finish a project and it's got a few parts of it that you feel really good about, I think that can be enough to be satisfied. You'll probably never be 100% fully satisfied with something, but that's okay, it just means there's more left to learn :) It'd be boring to make something perfect, right?

That's how I felt a few weeks ago. Then I submitted a game page. Now my motivation has skyrocketed.

Try talking to people about your project(s). You can derive encouragement from the positive reinforcement and constructive criticism.

I get the feeling. I waver between extreme confidence and extreme doubt in my work all the time.

The only thing you can do is keep pushing yourself to do your best work. Sooner or later doing your best will come naturally and you'll enjoy it- you won't even notice how much better you've gotten till you take a step back to have a breather.