First ever relationship!! Help please!

I've known this guy for a while on & off. Always had a good laugh with him, we always got on great when we saw eachother. He travelled a lot so communication would be rare to none when he was away. He has recently come back permanently. We met eachother and as always it was fun. We always had a lot of sexual chemistry & tension when we hang out. Thankfully we are pretty open so we had an open conversation about our feelings. We both agreed that there was some kind of spark there but both are not looking for anything serious. Long story short we both are open to casual sex - you know people have needs and all... I'm excited as I'm super confident with him but I have a few issues. Firstly I've only ever had sex once and it happened to be with him. I'm worried I'm too inexperienced for this relationship but I do badly want this. Also I am probably one of the most insecure girls out there. An inexperienced girl in her mid 20's, a size 12 UK, rolls here, stretch marks there, spots and all. The guy is very attractive and well built.

Ladies, how can I improve on my self confidence to go ahead with this & why is it that he wants me when he can have any girl??

I'm just worried if I don't get over these issues I will scare him or anyone else off. Because of my insecurities that's why I am so late to any kind of relationship with a guy and if this doesn't work out then I probably won't ever get a chance like this again. 😢