Slips out

Okay ladies -

I haven’t seen my boyfriend in over a month and a half and I finally will get to see him next week. Obviously we both know we are going to bang since we have been apart for so long. I need some advice on my back like if I’m on my side and he hits it from behind it slips out, if I’m on top it will slip out and same with in the bottom. Is there something I need to do with my back like arch it or not ? I just want to make sex enjoyable, I also hate being on top because I don’t know what to do, like what movement do I need to do or try to make it feel good other than just going back and fourth. Are there any positions or tips I should try?