Not good enough

No matter how hard I try, it seems I can never get ahead. Take my current math class. I'm trying my best. I feel confident that I'm doing the best work possible and that everything is clicking, then BAM! I get a test back and it isn't looking so good- like a bad car accident that makes others slow down to gawk at. I try and I try and I try, yet it is never enough. It then adds on to my internal hatred of myself and affirms that I am not good enough, will NEVER be good enough for anyone or anything. I will never be able to reach any goal because of some small thing that adds to the pile of other small things. I never seem to be able to dig myself out from under it.
I'm under a lot of pressure to graduate with all the credits needed to be accepted to grad school. I want to be a teacher, but all these little things are making me doubt my decision- yet again. I'm so tired of all the stress that I'm holding on to. This quarter alone, I'm taking max full-time credits. I was told today that I'm in danger of losing credit. When I was told this, I shut down. I don't know how bad it was or what was wrong with this test; I got angry and threw it away.
I don't know what to do anymore. I guess I'll never be good enough for anyone.
I'm not a good enough mother.
I'm not a good enough girlfriend/ wife.
I'm not a good enough homemaker/ housekeeper.
I'm not a good enough cook.
I'm not a good enough student.

I'M NOT GOOD ENOUGH.