I do suffer from anxiety and I play a lot of chess but ironically, I play chess to relax. To me, chess gives me a sense of control because all I have to do is concentrate on these 64 squares and I can ignore everything else in the world at least for a short time.
You should really take a long look at why you play chess and what you hope to accomplish. Why are you putting so much pressure on yourself? What is missing in your life that you feel you need to fill that void with chess? If you aren't a professional there isn't any reason to be stressed. It's just a game. If you lose rating points that's okay. You can get them back by playing more games. The way the rating system is devised, the more games you play, the closer you'll be to where you're supposed to be. If you aren't happy with where you're at then focus on getting better. Every game is a learning opportunity. Play games, learn from your mistakes, get better, rinse, repeat.
I would recommend taking some time off from playing rated games. Just play unrated and play ridiculous openings that are typically aggressive. Have fun with it and get back to why you started to play chess in the first place. I liked Hikaru's Botez gambit series on Youtube. Although, winning a queen down is hard, sometimes I do basically the same idea with a knight ie I play Nf3/6 followed by Ng5/4 and Nf7/2 regqrdless of what the opponent plays and then just play chess. You're obviously losing but it's fun to play and you get more compensation for the knight than you would probably think. I also like the englund gambit, 1.f3/1..f6 and 1...Nf6 followed by 2...Ng8. These are just fun openings to play and take you out of theory very quickly.
In terms of the anxiety itself. Focus on your breathing. There's lots of resources online about reducing anxiety through breathing but basically a long exhale will slow your heart rate and thus make you feel more relaxed.
Also look at your physiology and muscle tension. Tense and relax your muscles. Get up and move around. Anything that keeps your muscles loose. OTB if I'm feeling anxious I'll get up and got to the bathroom or something. Don't even worry about the time. Online, maybe play some relaxing music while you're playing.
Also, and this seems counter-intuitive but play a lot of blitz and bullet. This does several things. 1) it teaches you not to stress on specific moves ie even if you make mistakes you can still win. 2 ) Most of the stress in chess comes from time. If you had 6 months to determine a move it probably wouldn't stress you out even if you had a horrible position. By learning to play fast it will build your confidence that you can find good moves quickly. And confidence is the antithesis of anxiety. 3) Along those same lines, in fast time controls your opponents will make a lot of mistakes and it will build your confidence to find those mistakes. 4) The way to overcome a fear is to confront it and by playing fast games you are confronting your fears.
Lastly, choose a good opening rep and learn it really well. Something solid that gives you a lot of activity. The Tarrasch defense vs 1.d4 is a good example. It's a very active opening but simple to learn and you can just bust out the first 8 moves or so without thinking too much. That gives you confidence in the position and likely a time advantage.