You are not a horrible human being... Some thoughts and theories are exciting and the reality can be quite different. That doesn't make any of it right or wrong. Everything your feeling is normal.
Communication with her is a must. Honest communication about your feelings and thoughts and fears. When my husband had his first date with his (now ex) girlfriend, I left a journal out and wrote and absorbed and thought and felt everything I could. You can write it here, write it private, email it all to yourself, talk to her, talk to other's here or in your friend circle, etc... Repressing feelings isn't helpful.
There isn't anything wrong with asking her to remain at an agreed upon place (ie: no sexual encounters, etc) until your more comfortable.