I can't believe you made it as a US Marine and people are hassling you about it. Ridiculous! That's them and not you.
I'm really sorry to hear about your car. I hope you get the problems straightened out with your new one soon. It's pretty common to buy a new used car and find out it has a few things you need to fix, so you aren't alone. It's bizarre to give someone crap because their car needs a repair -- much more normal to be sympathetic, especially since we've all been there!
I have an 18 year old car that was paid off the day I bought it (used) 10 years ago. When people hassle me about it, I point out that I don't have any monthly payments. The car is getting old enough now that I actually get some compliments from strangers who once owned one and who are sorry they gave it up
With regard to feeling like you are surrounded by users and rude people, I think that growing up in toxic family situations has a huge impact. I am around your age and just figuring out that the reason I want to cut off all my friends isn't because I am depressed but because I am really bad at making friends. A lot of it has to do with the way I grew up and the relationship patterns my parents modeled.
So we can't really fix our parents (avoiding them helps sometimes!), but we can definitely start to focus on developing healthier relationships. Just being aware that we have a tendency to be drawn to users or unhealthy people is wonderful.
Are there any veteran support/service groups in your area? Any organizations that organize adventure trips for former soldiers? I wonder how it would feel if you spent some time with people who appreciated your service?