I was at the pool recently and the OW (it's 20 years ago and he's married to her now) walked past me towards the pool. I could feel the old anger and disgust (she'd been my friend) well up.
I felt sick, I'd have to share water and a lane with her. I could either leave- in which case she'd "win. Or, I could get in and swim.
I chose the latter and I swam like an orca! I out swum her, I swam and swam and hit the wall a couple of times at the same time as her but kicked off in front and kept swimming.
I was so proud of myself, I'd not let her stop my swim, I'd out swum her and I was fitter and faster than she. I know that after 20 years it shouldn't matter but- it does and I had to hold my ground (or water, more appropriately) I swam on after she'd got out so I didn't end up in the showers with her - I swam faster, better and longer than I ever do normally and I felt fab when I got out.
It's so hard when confronted with the OW, I'd like to say it gets better but frankly, in my case, it hasn't. But despite my desire to drown her, I didn't and I emerged with my dignity intact. I totally get how awful it feels when having to deal with OW, I've learned that keeping your dignity is important and if you can; show her you're better, fitter, happier, free-er or whatever.
Good luck and keep on keeping on. You will recover and find ways of dealing with these situations.