I'm so sorry you're in this headspace.
Emotional neglect does change the brain... But neuroplasticity also means that it is also possible to change it again in more positive ways. As you had many decades of your parent's negativity, it would likely take similar amounts of positive reinforcement to change back - so don't think it isn't possible just cos your odds are skewed at the moment.
The comments you are referring to - well, I might say similar things and so might my partner, because we're just expressing our own preferences. I love my partner deeply and sincerely, but that's the whole package of him, not that he is necessarily my ideal in ever individual respect. And that's pretty normal for most people, I think. Doesn't mean I love him any less or am any less attracted to him simply because there are also other things (including things different to him, physically or character-wise), that I also find attractive.
E.g. the clothes he suggests - maybe he genuinely does think they'd suit you. Maybe they really would but they're a style that would make you uncomfortable, that you were told something negative and untrue about by your mother. Who knows, but it's truly unlikely to be intended in the way you are taking it.
He can't tell you what positive effect he was hoping to have, cos he wasn't. But not maliciously, just that you are hyper-sensitive to such comments, while for many they're essentially meaningless "filler" chat, with no actual thought or intention behind them at all, and for sure no intention to wound.
He is sensitive to you, because he understood and stopped making such comments when you told him. But now you're ruminating massively on something that is hugely significant and impactful to you, but you're assuming it has the same significance and impact to him, when it really really won't have at all.
With respect, I sincerely doubt that other people see you in the way your mother did. What it does sound like is that, having had her views imprinted on you for so long, it's very much the filter you look at the world through and are looking for evidence of - "I'm not good enough" - even though there's lots of evidence to the contrary that your brain doesn't ever register because of your mother's toxic influence. Confirmation bias is real, and it's very easy for us all to believe that our spiralling negative/intrusive thoughts are "real" or "logical" or "true" when they're anything but.
Maybe your friend genuinely is sexy. If she is, that doesn't detract from you. There is someone more attractive than all of us out there, usually many people. And that they exist, or even that our friends and partners notice them, isn't the issue. The issue is that you still don't feel good enough in yourself, even though you are, and always have been, and your mother lied to you and abused you.
Like you say, all you can see is the "ugly child your mum saw". And you were never that. The challenge is to get yourself to know that, deeply in the core of your being, and fully reject your mother's hurtful lies.
I don't know whether therapy could help you change your core beliefs, but it doesn't sound like acceptance is working very well for you, and these intrusive thoughts are stopping you enjoying a relationship with someone who it sounds like cares for you a lot and it's trying to be sensitive to your insecurities.
Maybe EMDR and reducing the impact of those traumatic memories might help, or hypnotherapy or NLP with a similar intent.
You are not who or what your toxic mother said, and you never were. But what strategies might you use to help you come to truly believe that?