Can a girl get pregnant if I ejaculated in the condom then took it off and then stuck my penis back in her but didn't ejaculate in her unprotected. What are the chances?

I'm 19 years old. My boyfriend and I want to have sex. He is not a virgin, but I am. The only thing that I'm scared of is getting pregnant. We will use condoms for sure and as my boyfriend has had intercourse before, I'm assuming he knows how to use them. How effective can the condom be to prevent me from getting pregnant since I will be having sex for the first time?

Another issue that is freaking me out is what if the condom tears or comes off? Can I guarantee 100% protection and rely on my boyfriend's experience with condoms? I'm worrying a little bit too much, but I really want to experience sexual intercourse with someone I really love. I would really appreciate your help and advice because I'm in need of some.