tbh ive always struggled with orgasming so i feel for you anon, there is never a moment where im like 'okay i definitely just came' - - it's more like i have to guess, usually i just reach a point where i stop feeling good and it starts getting over stimulated instead.
my main advice is honestly not to worry about it too much. i used to get frustrated that i wasn't experiencing these amazing orgasms like most other women seem to talk about, but when i thought about it a bit i realised that i don't really care? like i know for a fact that there are times when i can still feel great and have so much fun during sex, it still feels good so who cares if i don't rreach definitive orgasm. at least to me it seems like you are so fixated on this problem that you aren't able to enjoy the rest of the experience, so iguess my advice is to just try relaxing and being more in the moment.
trying new things also helps! and if you are by yourself, just give yourself some time to relax and enjoy yourself because there really is no rush. sorry this ended up long winded and a bit generic, im wishing you good luck anon i hope it gets easier for you