First of all, thank you for serving our country. I'm sorry that you have to deal with PTSD and all that entails, and glad that David's lyrics were an inspriation for you to seek help. You are a hero and don't let anyone tell you differently.
To get to the topic of this thread-- there are several songs that have meaning to me.
First of all, "I'm Alive"... this spoke to me because all my life I've been rebelling against one thing or another. I had opinions I couldn't express because it wasn't "lady like"; I had different goals for my life than what my parents envisioned for me; I tried conforming, and that was a disaster, because I went through life feeling like I was faking it; and if I had to hear "why can't you be like your [younger] sister?" one more time, I was going to lose it! I live my life my way, and I've never been happier.
"Pain Redefined"... I especially identify with the lines "I stand and fight/ I'm not afraid to die" because I believe in the things I believe in so strongly that I would gladly give my life for them. Surprisingly, religion isn't one of those; but acceptance, individuality, certain social causes, fairness, and justice are among them.
"Ten Thousand Fists"... this speaks to me because there are so many people out there who are misjudged, misunderstood, and mistreated because of the music they listen to. This says to me "listen to me/listen to us, because we have something important to say! We may not express it in the same way as everyone else, but our views should not be discounted or belittled."
There are more, but those are the ones that come to mind at the present time.