There's a theory called self-efficacy. Self-efficacy is the belief that you can manage a task, in this case, getting good grades. Some people have low self-efficacy - they don't believe they can get good grades (which is where I'm guessing you are now) - or high, where they fully believe they can get good grades.
So how do you get higher self-efficacy? There are 4 factors that can effect one's self-efficacy:
Experience - You've experience that you can get bad grades and do well, which is probably the reason your self-efficacy has fallen. However, you've also gotten bad grades before (I'm guesing since you're talking about being more like your former self) - so focus on those experiences (I did it before, so of course I can do it again) and forget and forgive yourself for the bad grades.
Modeling - Self-efficacy can be raised by seeing someone else succeeding in what we would like to succeed in. Think of it as ""If they can do it, I can do it as well" - find people that inspire you and surround yourself with them. Enforce the belief that you can be just like them.
Social persuasion - Encouragement from other people. Surround yourself with people from your classes or elsewhere (even on reddit, like this subreddit!) who believe you can do it and who will encourage you and support you. Slowly, you'll believe it yourself.
Physiological beliefs - This one is kind of tricky, but basically, if you have anxiety symptoms and panic symptoms (shaking, sweating, etc), a person with low self-efficacy would tend to believe they are related to the bad grades (like "I can't do this, my body is telling me I can't do this") and it hinters their belief that they can actually achieve things. A person with high self-efficacy, however, would just say "Oh, it's just my body acting out, I can still do this thing perfectly well". If you have problems with this, it's probably best to discuss it in therapy :)
I hope those pointers helped a bit.