Recovery or Penance

I am not a great writer and am not sure why I am posting but I feel the need to tell my story even though I don't think it compares to some. Please humour me.
About 20 years ago I developed post natal depression, which went undiagnosed until things got so bad with my suicidal thoughts. I reached out to my GP who made the diagnosis and put me on medication. What a lifesaver that was. I felt great as the fog lifted. Problem was I never knew that the medication could send me manic. The new behaviour combined with the damage done previously in my relationship by the depression sounded the death nell of my marriage...not that I cared because I could do anything! Anyway not dwelling on the dark past I did survive and came back to a normal life, however I feel as if I have been doing penance to try and make up for all the sins of my past.
My life for the past 10 years has been dedicated to volunteering. It has given ne a sense of worth and value I could never get anywhere else. My big problem developed about 3 years ago when I was the first responder to a road crash call out. I arrived to find a young man about the same age as my daughter struggling to breath as his car had smashed into a tree. Well I held his head up so he could breathe and I tried to talk to him however he was unconscious and drawing his laboured breaths in. Further responders arrived and we rescued him out if the vehicle and transferred him to the care of the ambos. I felt relieved and exhausted. However my life changed forever about three hours later when I received a phone call that he had died. I never knew his name but in such a short time together I had given my all to save him and failed.
I still think of him and still come to tears. I have had nearly 2 years break from my volunteering and I miss the sense of value it gives me, however I am terrified I wouldn't survive another trauma. I am questioning whether the positive feelings I get from volunteering as a form of penance for my past can outweigh the new traumas I may encounter. I also wonder if my past predisposition to depression makes me more susceptible to PTSD. Anyway thanks for the outlet.