I have similar difficulties. I do extensive editing and proofreading.

Regarding its and it's. Rather than memorizing the rule, I think of it as, one word or two. 'its' is one word. Just the possessive form of it, no apostrophe. it's is a contraction of 'it' and 'is', two words. An apostrophe is used to separate them. Once I started thinking of it in these terms, I resolved that problem.

I also rely on spell check. It usually helps. Check out amitriptyline. My spell checker tries to suggest a strange correction.