Am I mental?

I guess I want to just spill all this out, and its not really for a response! But anything is welcome. I have been with my partner for a year now and it’s been an amazing year but obviously nothing is plain sailing over overall amazing! I have anxiety which isn’t something I like to admit, to someone who hasn’t experienced the feeling, i can feel it from something as simple as making a cup of tea, and the chain of bad events that could occur from just making a simple cup of tea, my partner is aware and has been very supportive in times of need.

Onto my story now! When we met, he had a solid group of friends which I admire! And one girl in particular was so close but I thought it was lovely as my best friend was a boy so I could understand the stigma that came with it, and it made me happy! But I noticed early on it wasn’t the same situation... they met 2/3 years ago, and both single she was head over heels for him, and he’s a picky guy tbh.. and never went for it at all, but they became friends, it was obvious their banter was very flirtatious but I thought it was too early to comment! After a few months I brought it up, and I got the background story which made me more nervous knowing she had past feelings but I thought, nothing I can do! Then she would kiss him on the lips on nights out, when I was there? And I would always mention the next day and he would agree it was weird but it was just friendly! She’s nice to me, in the weirdest way, any time I try to mention him, or say anything about him, she has to “one up” me like she knows him better! They cuddle and hold hands but again “all friendly” then today, I saw a message that I just didn’t want to see... the basics was she asked if I was staying at his flat, he said no, I was working late as I work 2 jobs, and she replied “good, just wanted to know the sleeping arrangements” and he said “fair enough” and she replied “oh I’ll be squeezing in for a spoon” and my stomach just turned into knots and I read on.. which hasn’t been my finest moment but what I saw was just.. horrible! The one message was “aww I was going to say I would rub it on you (referring to cream) but I thought that would be dirty” and he replied “I mean if you want, but I don’t think ally (me) would be happy about it” there was more but it would take me days to list them. I confronted him today and he’s said it was wrong but nothing was meant, what on Earth am I doing?