Oh God, I could have written that, and you have no idea how relieved I am that there's someone else who feels the same and has had the same experience as me. I am truly mortified about it so have NC.
I met mine in my late teens. We had an on-off thing going on for years that was really just about sex. He was pretty upfront with me about that from the start, and at the beginning, I fancied him but didn't want a relationship with him, so was OK with that. Then one day I realised that every single other girl he'd been involved with was a serious relationship, and he loved them. I started to question "why not me?" (although only in my head - I didn't want that conversation with him). All I could come up with was that I wasn't cool enough, clever enough, interesting enough etc etc etc for him. I knew I was attractive enough, but that wasn't good enough. I was about 19 at this point, and ever since then, I feel like everything I buy/do etc is done with "but is this cool enough/good enough/interesting enough/intelligent enough for him" at the back of my mind. It is RIDICULOUS because although I thought I was in love with him in my late teens and early 20s, I really really wasn't, I was just fixating on trying to be 'enough' for him. The groove in your brain thing that you mention really resonates.
I haven't seen him for 10 years now. I don't even LIKE him, let alone feel in love with him or even fancy him. However, I know that he is now married, and some stalking of his wife on Facebook reveals her to be everything that I always wanted to be but could never pull off. I know through friends that they have recently have a son. I always wanted to have a boy, but I had 2 girls with my (lovely) husband instead. My girls and husband are my life and I have never wanted to swap either of my girls for the boy that I originally hoped I'd have, but seeing that this man and his perfect wife have had a boy really hurts. They've even called him by the name that I wanted to call a hypothetical son (and it's a really really rare name, so such a gutting coincidence). Now rather than fixating on wanting this man (because I don't), I am fixated on his wife, because it feels like she's got everything I ever wanted - including the man (that I no longer even chuffing want! WTF?!) It's so destructive and pointless and I know it's silly but I can't un-programme my brain. Believe me, I've tried so many times.
Before anyone jumps on me, although I facebook stalk them, I would never ever even contemplate contacting them or stalking them in person or doing anything destructive towards them. I just wouldn't.
So, really sorry for hijacking, OP. I hope it helps to know that I feel the same, and my God it's been good to finally get that out there as I have never told anybody how I feel - not even my closest friends know.