@ggggggggggggg I am posting below a 10 year old thread of mine where I had the same dreadful fear. It may resonate as I recognise so, so much of your anguish from back then.
(At the time there had been a lighthearted thread about "favourites". It prompted me to brave my fear....to put it in black and white and expected a backlash. Instead the support I got on here was amazing. So it is here if you want to read it, and my journey to "fix" it. It is a bit long. It may not help. Or it may - even if to know you are not alone.
https://www.mumsnet.com/talk/relationships/2011250-I-do-have-a-favourite-And-I-feel-sick-about-it?reply=45413411
And, as an update, DD is now 18. And despite my fear 10 years ago, I now have zero doubts about how much I love her. For the amazing person she is. For her wild red hair and piercings. For the lovely relationship she has with her girlfriend. She is funny, and kind, and has a strong sense of ethics and morals. She is reliable and sensible at work and her dedication to her studies and her love of learning is just awe-inspiring. And yet she loves adventure, the outdoors, trying new things and is so unutterably brave. All mixed in with an amazing sense of the absurd and loves just messing around with her 15 year old brother. She is currently out at the pub with her friends celebrating their last day of school. I am so, so proud of who she is. From her inner soul outwards. (Also proud, boasting mum alert..she is about to sit 4 A levels, predicted A stars and As and then off to Uni). Right now, having re-read bit of my old thread, all I want to do is just go to the pub, pop in and just give her the biggest, biggest hug ever and cry all over her at how much I love her.
So that ^^ chunky big paragraph hopefully shows you you can get over this. You do love your son, but something is "blocking" it - through no fault of your own. Potentially the childhood dynamics. It was not fair, or right that you had to suppress your personality for your brother. Maybe this is coming out now? But remember, you are not your mother. You are determined not to do the same. That recognising it, as you have, is the biggest, first hurdle.
I did need therapy. But I get it may not be financially possible at the moment.
But as well as therapy, MN was an enormous support.
Please feel free to PM me if you would like.