Y can I only get my gf to come with my fingers an my mouth but not my dick?

Your dick isn't hitting the right spot in/on her. Try changing positions; even the slightest change in angle can make a difference. You need to find her 'G-spot' to excite her to orgasm. You may also be rushing things. Don't try soo hard for an orgasm; start by just finding a spot that excites her. Then experiment with your rhythm and pressure. GL.