Do you own any toys? Ever used any?
This is the first thing I would ask. Most girls with this issue have never tried them, and when they do, never go back. We can go from there 
Otherwise, you will have to explain to him that some women simply take a long frickin time to come, regardless of whatever you do. That's just how it is. The difficult part is bringing the girl to the point where she is comfortable to allow herself to come.
Remember that 90% of you having an orgasm is mental.
But my other question would be if there has ever been any form of abuse or serious relationship problems in your past?
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