You are actually answering your own question when it comes to "why" you are experiencing this challenge. A lot of this has to do with some deep fears you have when it comes to feeling safe when it comes to relationships. The fact that you have shared "if it fails it will be my fault", is something you were encouraged to believe in your history, possibly this comes from your parents or other areas where you were encouraged to "feel" that when things don't happen a certain way, it's your fault.

I have experienced suicidal feelings as well and often that came from being emotionally overwhelmed where I genuinely believed no one would understand it and suddenly being emotionally overwhelmed can most definitely be very confusing and scary. Also, ptsd can "magnify" emotional challenges and struggling with that myself I know how challenging that can be. When you struggle this way slow down and allow yourself space to create a safe place you can go so you have space to relax and give whatever is triggering you time to gradually calm down. It sounds like you did not have a chance to understand yourself emotionally where you can "self sooth" in a healthy way and build trust in yourself when it comes to working through your own emotions where you learn how to identify them and allow them to come forward and then pass and ease up.

Are you seeing a therapist you can talk to about this?