It's hard to tell just from your post, but so far it sounds like you're more of a "starter" than a "finisher" -- you're not following through enough on your projects to keep them going. I'm not an expert on your life, so this is just a guess.
However, the one that really stands out for me is: "Did another webapp for one of the student clubs. (Someone else also wrote one and theirs was better, so everyone (including me) just used that instead.)"
In your other examples, there were external factors that created large barriers to finishing the project. That happens sometimes, but with this one, you simply ... gave up.
There's not necessarily a negative connotation there; you could have made a perfectly logical decision after factoring in your interest in the project, the amount of work it would take to compete with the other webapp, and whether you had any immediate ideas that would make your version better. But, it looks like you're asking why you haven't been successful yet, and here's a case where there really wasn't anything stopping you.
Aside from that, I think most folks here have a pretty big stack of failures and dead projects. I sure do -- I've even lost track of them. I've got failures in inventions, big what-if ideas, political ventures, software projects, business leads, and on and on.
As long as you're able to get something valuable out of those failures, some lesson or some insight or some social connections or something, then it's not really a failure.
It's just more experience.