Agree with asking your GP, (although I haven't and probably should). I have the same thing, what I find often works is to slow down and try something else, e.g if I don't think I'll come through sex then stop and have a teasing handjob. I've also decided to accept it, particularly in shorter bookings, just enjoy the sensation.
I used to punt alot more than I do now and I'd do it to relieve stress, however worrying about cumming was another form of that stress. I now only punt when I'm relaxed and it's more enjoyable.
Finally the SP might help, some will up the pace and faster action might not help but many will have experienced this and want you to come. Rather than calling it a day maybe say to the SP you don't think you'll come through whatever and let them take charge. I used to see alot of Thai girls and in my head I wanted everything, after a while pumping away in anal and not coming I realised I wasn't actually enjoying it, the next few punts I asked them to lead and it was far better and I came more as well.
If I get round to asking the GP I'll post what she says, would be interested in anyone else gets advice.