First of all.. don't give up!!! Practice makes perfect.. and I'm sure he'll enjoy the practice...

I am 34 and have cum only once from a blowjob, and that was before I was married. Your husband should be happy that you are trying.. some guys wives won't even try....

I certainly am not an expert but a couple things that would help me to cum would be to make sure you stroke his cock in the same rythym as your mouth... and better yet, play with his ass... slide your hand under there while your lips encircle his cock... and be brave enough to insert a finger... I think I'd explode in a minute...