Dude you gotta go sloooooooowwwww. Start with a finger and lots of lube. Go up and down, in and out, and side to side to stretch and get her used to the feeling. After a while when she can relax around it consistently do two. Then depending on your dick do three. Keep adding lube throughout, this will lubricate a bit inside. Then generously lube your dick and try slowly. If it’s still too much, go back to fingering and try again. She might not be ready for it on the spot, that’s fine.