There are many sexual exercises you can do, not just by yourself, but with a partner too.
My favorite is this one, but be forewarned, it will create an EXTREMELY close relationship between you two.
It is a tantric breathing excercise. What you do is explain it to your girlfriend and then get her to agree to do this with you. It's a kind of bonding exercise, but having such intimacy WILL make the sex better than you've previously imagined possible in my own experience.
You basically are mouth to mouth [not kissing] with mouth open. You breath into her lungs, deeply and she breaths into yours, like when you're underwater and keeping each other alive by being mouth to mouth.
You do this for about 3-5 breaths then get some O2 and repeat.
This is a BEAUTIFUL excercise, and it might even work for gays, too, who knows?
Secondarily, this is a masturbation.
You get yourself to come near to climax and hold the base of your penis tight as a bastard to stop yourself from ejacualting. Keep on bringing yourself to orgasm, but whatwever you do DO NOT DARE TO COME. Just get yourself to come close to coming abotu 20 times, very close, but don't go over the edge and make seeds come out your tip.
After you DIDN'T come, go about your daily business. TYry to do this for an entire week, then jerk off in our girlfriend's face or up her as, to get ridf fo the stress and built up tension. Even after this you will prbably be able to go ALL NIGHT and THEN SOME and your girlfriend will be 100% willing to say "I love you forever"...GUARANTEED!
You can also do some yogic breathing to get yourself spiritual, too, so integrating sex safely into your life in a more wholesome way.
Hopefully you will take action and experience wonderful resutls, not just for yourself but for hers, too. | 
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