Hopefully you've had your HPV vaccines by now?
You're entirely right that this guy's being irresponsible, and you're right to be worried about STIs but there's a reason they call it "safe sex".
It is possible to minimise the risk of STI transmission, but it
is simply a risk associated with having sex. You're right: even when using condoms, STIs can be transmitted, but by and large, you're safe as long as you're responsible. Everyone has had a condom break on them, or know someone who's had it happen. The best thing you can do is educate yourself and know how to avoid it: it's still possible to have casual sex safely, and obviously the risk with a long-term monogamous partner who's tested negatively is incredibly small. I think yours is quite a normal "phobia" to have, but there's no reason to let this stop you from enjoying sex with a partner!
It may be worth seeing your GP about your pain: this sort of thing
does have a serious negative effect on people's sex lives, and if you're struggling to insert tampons, it may be a bad sign. Like Eliza mentioned, some girls suffer from severe vaginismus and struggle to insert anything at all, but it's something that can be worked on, and something sexual therapy can help with, if you want to have sex in the future and struggle with this sort of thing.