Don't play long games - at least not for now. I'm unsure as to how to advise a total stranger (or even someone I know) how to reduce stress. I suggest you start researching "stress reduction" by using a search engine and going to reliable sites such as webmd.com, mayoclinic.com etc.
Is this strictly a chess-related problem? ie you don't usually get stressed out about other things as well? If not you should consider getting a physical and making sure you don't already have high BP and/or any other physical symptoms caused by stress.
In the meantime maybe you could try moving up to 10 min games and keep on expanding your "horizon" for longer games and see at what pt they stop being fun.
BTW IMHO It's not normal to get stressed out frequently over chess. I get pissed off once in awhile when I bungle a won game or lose on a stupid blunder, but usually I can shrug such setbacks off and just tell myself to be more careful in the future.
I started on here about a year ago; played a lot as a teenager and was ok, and wanted to try and keep my brain ticking over now I'm in my 50s. Thing is, I mainly play 5 minute games, because they are quite fun. Longer games are far more interesting, but I remember now why I gave up playing years ago - it gets quite stressful, and gives me headaches! So is that just me or do others fine chess quite stressful as well as engrossing? Any ways of reducing the stress?