Re: distance varies with excitement level.

You are putting too much guilt on yourself. Almost everybody spank the monkeys. 9 out of 10, the other guy is lying! Many 3 to 5 times a week. Some 2 or 3 times a day. We knew a guy who did it 5 times a day. You are not the only horny guy on the planet.
This is pretty rugged equipment designed for heavy duty use.
However, the part I mentioned to you about overstimulating the nerves could be true in your case where just a little rest helps out.

Also, intercourse cleans the pipes better which may account for self-gratification leaving you with thicker semen where it tends to "gel" up from not being expelled completely. It is possible that it gets so thick that it STOPS. At this point, your body increases it's blood supply to your organs attempting to finish this job..leaving you VERY uncomfortable. This may explain your desire to cool your testicles/ pelvis often.
So...
I know you do not want to talk to someone about this, and you are shy, and afraid to talk about sex, but you should. A good urologist or doctor can check you out and see if there are any problems. Find someone you can open up to and get it off your chest. It also would be good to know if your prostate is healthy by checking your numbers, if he deems that necessary. This could save you health problems later on. Wouldn't that give you peace of mind?
Assuming he determines your semen to be too thick....
he may elect to milk your prostate/ seminal vessicles several times. (Ooooh God don't stop.) Equal to completely cleaning your pipes. Google search: "Prostate milking"
C'mon back!