Sex with my boyfriend is not that great.

Hey guys, so this post may seem like a whole lot of TMI, I apologise in advance, I just need some help!😢(I'm 18 btw.) So my boyfriend and I have been together for roughly 3 months now, and finding 'our groove' sexually has been rather difficult. The first time it just did not feel right, and it actually kind of hurt and felt like he wasnt 'hitting the spot' at all. I thought maybe it's just because I haven't had sex in a while and having only done it a couple of times before, I thought it's bound to still be uncomfortable. The next times it was the same thing over and over. I asked my sister and she said it may have something to do with his size (he doesn't have the biggest.🍆) So now I don't know what to do, and I don't want to bring it up unless I'm absolutely sure that's the reason it doesn't feel right, and if so how do I even do that without offending him? Please hellpp