I am in a relationship with my boyfriend at the moment and have been with him for 16 months. I haven't slept with anyone before apart from my boyfriend, and my boyfriend has tried having sex with this one girl and he says he couldn't get it in so he was still a virgin, but he has done other things with girls even though its not sex. How can I stop thinking of this? I don't want to ruin my relationship because I'm being childish.

What he did before you two started dating is none of your business and not your concern. Just remember that and forget about what he did. Being jealous of things he did before you is childish like you said.