I had a lot of issues staying right and like you id start to round it was a lot of mental stuff, your brain saying don’t do this you’ll break us. Once I learned to clear my head lock my lats and get tight the weight really started going up. I’m 6’3 265 learning to get tight also meant I needed to sink down a bit so working on my ankle mobility helped a lot. Another thing that helped me a lot were front squats I picked up some oly shoes and went to work front squating every day I was in the gym it didn’t have to be super heavy every day just something it really taught me to keep my core tight. It also fixed the large embalance between my quads and hamstrings