Firstly, some details. I used to be TBM, went on a mission, and came back after a couple months out. Left the church shortly after.

So I am currently attending USU, and at that age where all my friends are returning from their missions. I have a few friends that I grew up with that just returned home. They are still in their missionary phase, so they invite me to every church thing they can imagine. I want to just outright tell them no, and to stop asking me. The problem is, they're my friends, they always have been my friends. I want to do stuff with them, but whenever I'm available to do anything, they're always doing something with or for the church. Since they're my friends, I've gone to a couple church events with them (ward prayer, FHE, etc) just to hang out.

So what do I do? Do I stop hanging out with them all-together? Do I just tell them I don't want to hang out with them if they're doing church stuff? Should I just keep going with them? The problem is, almost everybody up here is Mormon, so it's hard to make decent friends that aren't Mormon. Just frustrating.

I'm not sure what the answer is in your case, but I do understand the problem with all your friends being Mormon. I joined the Church as a teenager, so every friend I have is Mormon.

I haven't made any new friends outside of the Church, but the reality is that I really don't fit in with these people anymore.

I guess the only thing you can really do is to try inviting them to non-church type activities. Let them know that you're really not comfortable doing church things, but that they're your friends, you love them and want to do things with them.

You may need to be the one to come up with activities that you would be comfortable with. Just a warning though. You may end up discovering who your real friends are and others may fall off by the wayside.

You're in a pickle.

So the fact that you "came home early" from your mission means that people are going to assume that any "falling away" you do is because of embarrassment or sin, not because you've discovered the truth.

Because of that, I would recommend being honest with them.

Don't be preachy/pushy, just say something like:
"I've done my research and come to the conclusion that the church isn't true. As such I'm not going to go to any church functions, but I wont dissuade any of you, and we can still hang out."

Most younger people (I am 29) are realizing that there is a decent chance that TSCC isn't true (but they wont research it because that's scary). So even if they are TBM, if you are up-front with them they wont shun you. Older TBMs seem to be more into shunning.