I have had sex with her (many, many) times before. But sometimes I'll be putting the foreplay moves on her and she will just constantly resist me progressing to sex (like prevent me from taking off her panties or something). I know she's not on her period during these times and 99% not cheating.

Option A is to try harder, like rip them off and be really aggressive. Or instead, when she says "not yet" like 10 times and wastes my time for like half an hour, option B is to show her that she missed the opportunity and that I don't need sex like that. Just drop it, get dressed, and do something productive.