Can’t cum

So like I don’t know how to explain this I’ve climaxed hand free from a few bjs and even a little when bottoming but when I’m topping and getting like a bj i feel like I’m gonna bust like seriously but then nadaa lol my dick says not yet but I feel like I almost did bust any pointers maybe advise? I am a bit on heavy side but almost 30 any one have this problem sometimes?