Help! I'm terrified of ruining this relationship! im considering counselling!?

You shouldn't blame yourself for feeling that way. If my boyfriend had slept with other girls before meeting me, I would probably be going through the same thing you are. I have to be with a virgin, or it's just too weird for me (I'm a virgin as well). You love him, and hate the thought of sharing... that's understandable. But you just have to think about the other girls as ancient history. He doesn't care about them anymore, he cares about you. It might help, once you get older and you two are still together, to move away from there, so you won't have to see them and be reminded of his past. Seeing a psychiatrist might help you as well.