I'd actually seduce my university crush (god there were so many opportunities!) rather than find out later he was gay too.
I'd go out with friends to clubs & bars & things even though I didn't (and still don't really) drink. When I recall the Scandinavians who asked me to join them... and I didn't... what was I thinking?
I'd come out earlier - and hopefully avoid hurting all the people who were in love with me, with me oblivious to it all.
I'd do some more serious swim training early and get ahead of the pack then rather than working my way there now.
4 hours ago