I think what you are experiencing is actually normal, because you lost two people that you were very close with within a five year period and your early 20s are a very difficult and trying time for anyone, trying to find their place in the world while making a name for themselves and achieving stable employment. You got a degree! Good job! At 23, that's great! Don't feel bad for breaking down in front of your girlfriend, for what it's worth, I'm sure she appreciates the fact that you can do that in front of her and you're open enough to express yourself. Like I said, this age is a tough age for anyone. 18-25 are usually the most stressful ages because you're just starting out in the real world and having to take care of yourself, and fend for yourself.
You're on the right track, just keep doing what you're doing, get into a nice rhythm with work or continuing your education. Take each day at a time. Focus on gratitude, and appreciating what and who you have in your life right now, and remember all the lessons your loved ones have taught you thus far in life. And remember,you are not your thoughts.
Thoughts come and go, they are fleeting. They are just your brain's way of trying to make sense of the world around you. Do not cling to your thoughts and make judgements about yourself because of them. Actions are what make up a good man. Your actions and how you treat others, that's what counts. Whenever you have negative thoughts, replace them with calming, amazing, beautiful thoughts instead just because it feels better.