I was a police officer for 13 years and an EMT for 10 of those. I had a 2 year old girl die in my arms on the side of the road, which is what prompted me to get EMT trained, so I know exactly how you are feeling right now.
I can tell you that I worked on countless trauma patients with every type of injury you can imagine. Every single one of them that was core 0 and required CPR died anyways. That is not unexpected. CPR rarely is succesful unless the stoppage is due to a heart attack or electric shock. The only "save" out of 30+ applications of CPR I had was for 1 heart attack victim.
I guess my message to you is: It is excellent for you to advance your first aid training and carry a more comprehensive first aid kit. It will help a lot if you need to deal with minor issues. But for major trauma the chances are very slim your kit will make a difference. I only came to that conclusion after going through extensive EMT training and carrying a full jump kit for years myself.
Learn the basics: Stabilize the patient in situ and don't move them unless the car is on fire. Keep their head still and manage their airway. Put direct pressure on severe bleeding. Talk to them even if they are unconcious and do your best to keep them calm until advanced life support units arrive. Even though Colorado is a Good Samaritan state, which theoretically protects you from liability if you try to help someone, I would not advise trying to use any advanced airway management tools unless you have certified training in them. You may win the lawsuit but you might still have to suffer through going to court.
The other thing I gained from the training was the knowledge that I did everything humanly possible and there was no reason for me to feel guilty because they didnt make it. Lots of times these folks could have this same accident on the steps of a level III trauma center and they still wouldnt survive. Its good you're doing things in response to your experience but dont beat yourself up over it.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang