When you suck at something, you've probably convinced yourself to keep going for one reason or another. We tend to get stuck out of habit, but you just need to quit sometimes. As venture capitalist Brad Feld often says, "quit what you suck at".
He explains why in an interview with productivity and ideas blog the 99u:
If you are doing something poorly AND you don't enjoy it, then you probably suck at it. If you are doing it poorly, but it's important to you, or you want to get better at it, or it fascinates you -— keep trying. You usually know when you hate something -— that's a leading indicator that it's not worth doing, unless you have no other choice.
You are, however, going to suck at most things the first time you try it and probably for a little while after that. This advice wasn't crafted to tell you to give up every time you suck, but rather to pay attention to what you hate and what you aren't good at. When those two things coincide, and you've given that line of work/activity/whatever a decent change, you should quit, or you'll continue making yourself unhappy.