I know this feeling. My ex (also my first sexual parter) was so small that I sometimes couldn't tell if it was in, or just rubbing against me. I know it's the worst question you can ask a man.. but I had to!
I know how horrible it is to be really, really horny and just need a good shag (sorry!) and then to have that dangled at you.. and you're just like.. okay.
Yeah, you can use fingers, mouths, toys.. but IME nothing quite matches up.
Because he was my first, I sadly didn't know any better.
My other sexual partners have been of varying sizes, and if anything my partner now is a little TOO on the large side haha. From one end of the spectrum to the other, but so long as he's gentle, it's okay.
For me, now, it would be a dealbreaker for me, as I honestly think that sex is important. I didn't realise this when I was with the small partner, as I didn't know any better. Now, sex is good, and I went through many years not even realising it could be this good! I know that probably doesn't help you very much, but if it was me, I would have to end things.. I'm sorry.