BJ 101: Help!

So I have only given oral to 2 guys...both different in size... current guy is bigger...he likes to grab my head and push himself deeper...and causes me to gag...and I have never made him cum...I suck and go up and down...I play with his tip and circle it with my tongue. What else can I try? I have played with the skin portion from where he was circumsized...but I still can't...I can feel myself get frustrated and I don't think that is helping...what can I do?