Uncomfortable penetrative sex as a beginner

I'm 19F and I have an older sexual partner (It's just sexual).

So, we've been having sex for about 3 months now and everything is amazing BUT penetration. I enjoy making out with him, oral sex (giving and receiving), but penetration is always uncomfortable to me.

I don't know if it has something to do with it, but I am quite petite (4'11/89lbs) and in contrast he is a giant (both in height and size down there) + He's quite rough + he's my first experience with penetration, so I have nothing else to compare to, that's why I came here to ask for your help.

So I'm wondering if there are any tips to make it easier for me or any advice in general. Thank you 💖