I am trying to brush up my maths after an initial bachelor in STEM from a no-name school with piss-level calculus courses and I am getting blocked on every sequence and series problem.

We learned only how to do the computation part and were never taught how to actively think and produce proofs. No one from my class knows what epsilon-delta stands for, or what inductive reasoning is, and the squeeze theorem might as well be a kama-sutra position.

How do I get better at this? I want to improve, but I feel so powerless and stupid...Pic related is a simple problem that I can't even do unless I put 20 minutes of effort into it...Have I reached my limit or is it even possible to work hard enough and finally improve?