I Suck at Sucking 😩

I feel like I used to be really good at giving head but I've been with my boyfriend for almost 3 years now and lately I just haven't been very good at it. I keep nicking him with my teeth. That's never been an issue for me in the past but lately I don't really want to suck him much anymore. I used to love doing it so much I'd just offer it up and not even want sex or anything in return. Now, I'm a little embarrassed and worried before I even start that I may bite him. Honestly he is way girthier than I'm used to so it can be difficult at times taking him all in but in that past I didn't have this much trouble. Not sure if its just self esteem or what. I have been feeling down on myself lately for my past. I feel like I was a bit of a slut and I feel some guilt about that. Does anyone have tips that may help me do a better job, with less teeth? Or just tips on how to relax more while doing it?