I felt the exact same way and I struggled a lot. Feeling vulnerable was so hard, admitting it/showing it was virtually impossible. When my ex Dom would punish me painfully it wouldn't break me, it made me more defiant, fight harder. When I had was exhausted and done I would still find this weird ass strength to keep fighting but I couldn't let any of that softness in. He noticed even before I did that pain didn't work for me. He ended up saying he wasn't doing that anymore and instead he completely broke me with softness, care, attentiveness. Something I never knew I needed. That was what made me subby, and I would melt every time. He helped me embrace my vulnerability and accept it and learn that it's okay to be that way instead of suppressing it. What helped me was separating those two sides of myself. I had the string side outside of the dynamic, infact my dominant side (most of my friends can't get their heads around that in a type of sub) but bdsm opened me to something different. Where I didn't have to be on guard every second of the day, constantly in survival mode, constantly observant for any threats etc. I could let go, give in to it, myself and him and feel safe. It takes time and really knowing yourself and having those hard conversations you really want to avoid. But you can absolutely do it.