Well maybe your size has something to do with it. My husband is bigger around than my mouth can accomadate and it hurts my jaw a lot. The more often I do it, the less it hurts, but it is definitely a lot of work for me. Also, if you moan while she does it, it will let her know she is doing a good job and she will want to finish for your enjoyment. Talk to her and let her know how close you are, so she will know how much longer she has to go. Also keep your hair neatly trimmed down there, it cuts down on odor and tickling of the face!!
Good Luck. I know this can be a big problem for a lot of couples, so you are NOT alone