You can read as many textbooks as you want, but if you aren't constantly testing yourself (Questions related to it, mainly understanding & trying to answer without any assistance at first) wouldn't make it worthwhile.
In Math degrees (I did Physics-Maths), you are basically working on the way you think. You become more aware of this state of "thinking" (You'll more often speak to yourself, in your head - becoming a more logical person who thinks before he answers) - it usually means you are becoming a thinker on your own, and working on solving problems that are getting harder & harder.
Maybe the reason you can't seem to remember formulas & methods is that you are using calculator websites. You have to give that dedication.
Like a teacher of mine once said: "Well, the calculator websites will not do the exams for you".
Go subject by subject, go on your weaknesses. If you have time, dedicate it to making your brain work & try solving the problem. That's the only way you'd be able to figure out what is that slows you down.
I'd usually try to derive the formulas on my own, it helped me to understand why I do what I do (Especially in Physics, it requires a lot of work).
When you are answering something, try to give a brief explanation (1-2 sentences) of why you did what you just did, step by step - that's what I can say from my own experience.
Good luck!