@clepsydra12,
Try asking your partner to guide you in with their hand. You could be aiming too high & pressing it against her clit, when you need to be aiming a little lower. Try rubbing it up and down her pussy, using your hand to guide it, & you should find the right spot & your old boy will slip in. It could be she is not wet enough, in which case a bit of lube will help. Have you discussed this with your partner/s?
When it comes to girls being too tight, that is possible if they're fairly inexperienced, but the vagina can stretch to push out a baby, so unless you're King Kong, it should fit fine as long as they're turned on enough.
Bar posting photos, it's hard to say if your junk is 'normal' but you can google pictures & see if it's roughly like the results. They do vary in terms of size and can bend in slightly different ways, but should all function in roughly the same manner.
This feels strange to ask, but is your cock particularly big/small, thick/thin? (Presuming the average is 6-7 inches erect & 1-2 inches wide)