I would second nearly everything that has been said above, and add that part of being a professional pilot beyond the technical aspect is knowing how to deal with one's limitations.
So you got a bad mark on an assignment and a course. Lots of pilots have a subject that is beyond them. I teach in the sim all the time and come across professional pilots who have a very limited understanding of say, electrics or hydraulics. It's all pure magic to them. But they don't need to know how current flows through a wire, or that when current reverses that's a bad thing. They just need to know that when this indicator reads 0 and this one reads 32, that's a bad thing and if that bad thing happens, do this checklist.
You are going to fail at something in aviation. In fact, you're going to fail at a lot in aviation. Everyone does. Everyone has failed an exam, failed a flight test, or did something inside the airplane they're not particularly proud of. Some have done all three, and yet they're some of the best pilots I know. They're not the best because of their mistake, but because of how they handled themselves during and after. They learned, adapted, moved on, and shared their experience so someone else wouldn't go through the same thing.
This is going to come off very harsh, but if you cannot do the same and you would prefer to wallow in self-pity, then this is not the industry for you. If, on the other hand, you can get over your mistakes and turn them into a positive, then come on in. This industry has a very high barrier to entry and there is no sense in making yourself one of those barriers if this is truly what you want to do.
You have a bunch of professional pilots telling you this is not an extremely difficult career and that you're overthinking this. So now, the ball is in your court and you have no one to blame but yourself if in 50 years you're playing the regret game because you let two bad marks define your life.