You are lucky to be female as you have a backup system. Try taking it anally as it will likely hurt less, then work yourself up to using the vagina again. If neither works, seek medical advice, or date a doctor.
Lots of lube; maybe try a few different size dildos; start with a smallish one then go larger and larger until you feel thats all you can handle? I don't know, just my opinion.
Plenty of foreplay, have the guy make you cum with his mouth 1st and have him work your pussy with 1,2,3 or 4 fingers at the same time. Your pussy will become "tented" and you shoudn't have any more problems fitting him in.
Good luck.
First you need to feel you can trust your partner.
Second learn to control the kegel muscles, once you can relax and tighten them you will be able to play with large dildo.
Third Forplay is a must the hot and horny you are the better.
Fourth LUBE my preference is sweet almond oil. You can get it at most health food stores.
And Lastly we all have our limits and if we push it to far we can end up in pain.