I see no reason why you couldn't pass exams and be happy if you didn't have all these problems.

If I were either on drugs, on heavy medication, or depressed, I would also fail exams.

I mean, some people are depressed even though their life is going perfectly. In that case, I have no good answer. But if things are rough and you feel bad, that's how it ought to be. I know how it feels to be really depressed. But there are people whose life is as perfect as it could be, and they still feel terribly depressed. For them I have no good advice.

Your life sounds like it has a lot of potential for improvement. I hope you see you are in a vicious cycle. You feel bad because you do bad in college, and you cannot possibly do good at college because you are feeling bad. I hope you also see that this is not a permanent thing. I hope you can see that the level you are performing at now cannot possibly be the peak or limit of your ability.

You are at an age where psychological problems aren't uncommon. That means that when you are older, regardless of if you are successful in your current goals, you will naturally start to feel less bad. If that means you will automatically also be really happy, I don't know. It might be that your affect mellows and your lowest lows aren't quite that low.