You are completely right.
But, when you graduate those grades don’t matter anymore. You have a degree, and although great grades are a matter of self-esteem, a degree is a degree is a degree. And that is what truly matters in the grand scheme of things.

If you’re upset with yourself for not achieving a goal, instead of beating yourself up over it look forward. Ask yourself, what can I do to be happier next time.

That question gives you many options. If you truely wish to get a better grade it leads you to ask what you can to to achieve that goal.

On the other hand if you wish to learn to accept that level of performance then you need to ask what you can do to accept such a performance level.

I hope you decide what is important to you and work towards being happy with the results.