Here's advice from a guy who has been making love karezza-style for years:
I think you will find that the discomfort will pass as your body reorients itself to this new way of engaging sexually. You mentioned you have tried cold water and breathing the energy up with some success, here are a couple of other tips:
One, if you find yourself getting close to the edge while practicing karezza, cool it down a bit. Enough to keep your erection but not so much that you get heated up. Stay in the middle zone.