How do I control my emotions?

Hey there. I'm new here and I really hope being here can help me reduce some of my anger/anxiety/depressing thoughts..

I'm having trouble controlling my emotions regarding my grades at university - well not only about grades but this is what's bothering me this afternoon..
I put in a lot of work to pass my exams, I try to be social, which never works though, and when I finally think I did good I either fail my exams or barely pass them.
Every time I sink into this deep dark hole where I can't seem to find any light whatsoever to help me guide my way out.
I get angry at myself, the other students for getting better grades, I get mad at my professors, the university, my girlfriend.. And every time I want to quit studying.
However, quitting is not really an option. I have chronic back pain and can't find a job that suits my pain, so I wouldn't even know how to pay rent if I gave up my education. (In Denmark the government provides enough money for us to pay rent and food while studying).

I'm currently seeing a psychiatrist. For the past year we've been trying to figure out why I constantly feel depressed and have terrible mood swings. About 6 months ago she told me she's convinced I have ADHD and Borderline personality disorder.
However, currently she's just treating my ADHD symptoms. She's told me she wants to treat one thing at a time, even though I went to see her for my terrible mood swings and so far she's just treated my non-existing ability to concentrate and focus on anything. Anyway, that's a completely different story...

So.. How do I stop these frustrating thoughts about quitting my education and how do I learn to accept that I am just not good enough?
The reason I'm saying I'm not good enough is because I'm always the one with the bad grades.

I'm sorry for the long post but I'm really at a loss on how to control my stupid feelings.

Is there any possibility of back pain relief for you? I had pinched lumbar nerves that drove me crazy with pain for over a year until I got surgery to make room for the nerves.
I began with a primary physician's referral for a chiropractor who gave me a back brace and TENS (electroshock) unit. It felt good for about 20 minutes after a session, then the pain returned.
I went to a spine/pain management center who sent me to physical therapy (PT), gave me x-rays, MRIs, began a regimen of steroid shots and put me on meds. Nothing was working. I was beginning to limp and stumble at work.
My last ditch was an appointment with a neurologist. He looked at my MRI and said "I think I can help you". I almost cried with joy when I heard him say that.
He did a Lumber Laminectomy to make room for the pinched nerves. That night in the recovery room, I noticed that I could feel my toes again for the first time in about a year and the pain was gone. My life is much better now and I feel like a new guy.
Living with chronic pain makes us feel grumpy and old. If there is any way that you can get relief from that, it may help clear up a lot of the 'emotional' issues.
Beware of having to depend on painkillers. They can have side-effects and lead to dependence. Watch any excessive alcohol, caffeine or sugar intake. Be good to yourself.
Best of luck to you.