Pressure during sex and PAIN

I’ve recently got into my first proper relationship and we have had sex a few times but not many. (This is my first time having sex whatsoever). As soon as it goes in I feel this strong pressure low down (like my organs are becoming rearranged?!?😂) that is obviously not the case but that’s the only way I can describe it.😂😂 And this dwelling peeing feeling at the same time. I’ve tried to relax and that’s helped slightly sometimes but the sensations just come back. I feel bad because I have to tell him to stop, and he’s not fully in. He’s quite well-enlarged and I’m thinking that may be down do something? I’ve read that the peeing sensation may be down to “squirting” but I’ve been finished non-penetratively and never had the wee feeling. It can be uncomfortable and painful so how can I get this to stop xxx