Well you are not alone. Being the least favourite or trouble child stays with you and causes you to be an overly anxious and unsure person and usually a pleaser. It makes you wonder what was wrong with you and why you didnt get the same love. It took me a long time to see it but once i did i could learn to understand me and manage more in the world (the extend of my emotional neglect was quite severe unfortunately leading to quite severe mental health problems) I find it funny when you are accused by a parent of being an overthinker etc, well duh you were supposed to nurture me you made me overthink like this!!
I have set boundaries and have a relationship with my mum these days but i no longer hold back on how i feel and will challenge her comments. This has reduced them but she must still have them. The funny thing is now i am grown and relatively stable she occasionally tells me i am now her favourite. I tell her straight i don't like it my behaviour, nature and achievements should have nothing to do with how she views her children. We are all worthy of unconditional love otherwise why bother producing us.
Sorry to detract from your current problem i just wanted to share what i have learnt about this kind of situation in the hope it makes you feel better. You are perfect just the way you are, never let someone make you feel you are not