I appreciate your and other commenters suggestions, but I've been down all these roads. I spent a whole summer slowly "working my way up," 5 mins a day the first week, 6 mins a day the 2nd week, 7 the 3rd, etc. It doesn't matter. As soon as I hit 12 minutes once, I turn bright red. I can do 11 for as many days as I care to and my skin looks like I've not been outside in months. I know it's not just subtly changing either, because I don't end up with tan lines even though I always wear the same basic swimwear.

I've also tried dminder, low SPF, all kinds of things. As far as I know, there isn't anything congenitally "wrong" with me or anything, but I wouldn't know what to look for to rule it out.

I even had someone tell me the GAPS diet would fix it, but I did that for two (very boring) years, and it didn't help either.

It could be that it's just hopeless, but it's bizarre to me that I should end up so evolutionarily unfit for my home planet.