Im plus and my gf is plus size. why cant i seem to get in there to have . Is it my size, or body type?

I really want to do it but everytime we try i cant get in there. I think my isnt big enough she says its hugh and other women ive been with said the same, but i cant seem to get in there. Its kinda embarrasing. Cant someone whose is plus size tell me what to do. Even if know not plus size give me some advice plz. I wana mak her so happi. I know isnt everything but i also know that a man making good love is beautiful so i really want that for us. thanxs