are you masterbating too much? this may be the case. you may be desensitizing yourself. stop touching for a week, have her then do what she does and then see if you cum.
If it takes you that long to cum, how the fuck old are you, father time?! Try deepthroating your g/f. Just shove your dick down her throat hard, and fuck her mouth. She'll probably pass out, but oh well, it'll feel mighty good, old timer...
a very good random number!