I've had the same problem. I get a little frustrated too, because I know she's trying hard. I think Sammy73's advise is good. Some lube and a little coaching on her technique will go a long way. Ther'e's some good advice on differnet websites, but you've got to filter though all the other junk that will come up if you do a general search.
Oh, some other advise a friend mentioned to me one time, just don't tell her "this is how Mimi and Jade's Oriental Massage does it..."
Try using some sort of lubrication - that can enhance the sensation, which may help arouse you and help you reach orgasm. You can buy sex lubricants, including some that have a tingling sensation, but normal moisturiser can work just fine (and some of those have a 'tingle' ingredient too).
I'd guess that you've just gotten very used to how *you* masturbate. You could ease off solo for a while, or try assisting her (start yourself or take over for a bit, then switch back). It doesn't sound like you need Cialis or Viagra - not if everything is good.