I (20M) have been emotionally invalidating my partner (20M)

My partner (has OCD and CPTSD) struggles with really low self-esteem and a traumatic household, I (have ADHD) try my best to support him but whenever we have a disagreement or argument he tells me I don't really understand him, that no one does.

He holds really negative views about himself and whenever he expresses them i tend to tell him ill be by his side and that even if it all seems hopeless i believe in him, but ive come to realise that it just makes him feel invalidated. I really struggle to hold any of strong or negative emotions because no matter how hard i try i fall into the same tendencies of fixing or solving it.

Due to my past and experience with my parents i've developed a bad habit of telling white lies, it doesnt have to be for something big, in any scenario where i feel like bending the truth will make the other person worry less or that i can be sure they wont be upset at me. Even when he's sharing his feelings i tend to guess what he might say, like if he says 'ive been really sad since i got home' i immediately jump in to say 'yeah because of how stressed youve been with the cat and the money'. I also tend to mask and start automating my responses to what he says, i begin replying with 'oh im so sorry honey' 'yeah i understand i know'.
Its like i cant stop putting on this performance of a well behaved, perfect and caring partner, i want to stop. Ive read up on similar posts about this kind of situation and almost everyone says that being in a relationship like this has made them feel so lonely.

I realise that I lack a lot emotional capacity, I find it hard to be honest about myself and my opinions and have strong people pleasing tendencies but I want to grow. I really care about him and I feel worried that I am not able to connect with my boyfriend emotionally the way he really needs. Have you guys gone through something similar, in friendships or any relationships? Because i truly want to learn to validate my partner emotionally, and i don't understand why i find it so hard to let any emotions in or through.
I want to know how can i stop my habit of solving the 'problem' and learn to sit with discomfort.
Any kind of advice in this situation is helpful, I want to make him feel valued and understood.

TLDR: I keep invalidating my partners strong or negative emotions with automated responses or with my inability to stop myself from fixing or helping him. I struggle to sit with any painful emotions and while ive been trying to reflect on those traits i still cant validate him. He says ive never really understood him even if we both love each other.