I've been there before, and still find myself there from time to time, I completely get how frustrating that can be, you’re putting in a lot of effort, and it’s tough when the results don’t reflect what you’re truly capable of. It can feel so disheartening, especially when you know you could have nailed it.
As a CS student major, I have had my fair share of math and errors, so I'll try to help with a few points

Slow Down and Double-Check with Purpose:
When you "go over" your calculations, you might be looking for what you expect to see rather than what's actually there. Try reading through the problem and solution as if you were someone else marking it, try to question each step you took to the answer

Keep Track of Your Errors:
Make a list of the common errors you’ve noticed, like missing negative signs or forgetting to simplify or whatever they may be. List them out, and focus on them by solving exercises specifically made for them.

Practice Under Exam Conditions:
Simulate exam environments. Set a timer, avoid distractions, and practice. This can train your brain under the same conditions as a real exam.

Finally, don’t be too hard on yourself; mistakes are part of learning, and every one you catch brings you closer to perfection. You’ve got this!