You CAN get real pain from focusing on a body part. I proved that to myself when I had pressure pain in my stomach for THIRTEEN months on a daily basis! It finally got to the point I couldn't think about anything else anymore and I gave in and got an endoscopy (I DON'T go to doctors for things and I NEVER get tests, they scare me TOO BAD!!) But I had to, I couldn't deal with it anymore. The DAY of the test when they told me everything was fine, that pain went away and never came back. So your mind can definitely create REAL pain.
Don't cancel dates - force yourself to go, you have to "get out of yourself" You will sink deeper and deeper into a hole. You may not feel like going out, but GO. Don't talk about your health at all to the girl, if you act fine pretty soon you'll have moments of clarity were you will actually forget. You can't FORCE yourself to forget or "not think" about yourself, but those times will come spontaneously, then when they do you will say, "Hey! I was really good for a while!" (at that point your worry WILL return and so will the symptoms because you thought about it), but then another moment will come, it will last longer because you will realize you MUST be OK. You'll then get to the point of being YOU again. But you have to START by forcing yourself to get out. See the girl, see your friends, STUDY.