I agree with Sebbie, buy some speedos and wear them under your shorts while running or heading for the beach. Get used to having them on and around you. Wear them in the shower at home, after your run, you speedos are supposed to get wet ad you may enjoy them wet tightly clinging around you.
Start running in the shortest shorts you are comfortable with (your speedos hidden underneath of course), then when those shorts feel almost normal buy some an inch shorter and get used to those.
Why stop at one pair of speedos, start a collection, your first pair will probably be black with 3" sides, but how about 2" sides or even a 1" side bikini. Then there are other solid colours and printed patterns to explore.
You say that you have always been lean, perhaps you are even skinny, your PE teachers looking for muscular explosive sprinters for their football teams may have been sarcastic about your efforts, the school bullies would have picked up on this and reinforced the message breaking down your body confidence. On reaching puberty you may have had to hide your developing sexuality from those that were already abusive about your body.
Slim young men are beautiful but often need someone to tell them so after all the bullying and jeering they go through at school. Wear you speedos with pride, Tom Daly did and he won the World Championships and got an Olympic bronze despite bullying at school.
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