Slow to cum and usually not at all!

My girlfriend and I have been having sex for a couple of months
now and when we do sometimes it takes me a really long time
to cum and usually not at all. I know it bothers her because
sometimes she asks me to cum and I can't. But I don't
know what to do. If I masturbate I have no problems getting
off. Only during sex, and it doesn't matter wether
it's vaginal, anal or oral. And it's not that I
pace myself either by any means. Any advice on how to correct
this would be greatly appreciated.

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