Blow job advice

So I never have liked giving blow jobs but my husband always wants one. He says I do a good job but everytime I do it my teeth hit my gums on the inside of my lips like the top and then its sore. It's hard to explain but its almost like a cute on my inside of upper lip after that hurts. Idk if I'm doing something wrong.

Also once I was sucking hard and his dick was hitting the roof of my mouth and I was sucking that way.. hard to describe.. anyway he said that felt the best but after I had blood blisters on the roof of my mouth so I dont want to do that again:( ugh I feel like I'm doing it wrong.. help.