How to stop my gag reflex while deepthroating?

So me and my boyfriend have been sexually intimate for a few months now and I've tried giving him blowjobs by following threads on this sub, but he doesn't orgasm until he is deepthroated.

The problem with deepthroating is i start gagging whenever I do that, not because I don't want to do it but because I can't breathe, i try to breathe through my nose but it doesn't help much.

A lot of times I'll start giving a bj and my boyfriend just grabs my head and starts shoving it down my throat at his own pace, which does make him cum so I feel good but sometimes i just can't keep up and feel like throwing up and have to stop in the middle and just breathe for a few seconds.

Another problem is, his dick is kinda girthy and sometimes he feels a bit of teeth which can be painful for him :( How do I stop this?