Okay, so while not the exact situation, I had a similar situation to you when I was in my late teens/early 20s.

I was dating a girl who was a very high achiever. Constantly working hard for school, got into an excellent college, volunteered a lot, etc; Me...failure to launch. I went to college briefly, but I was having a hard time finding my place in the world. I wasn't sure about who I was and what I wanted to do, and I really didn't have things figured out for some time after that. A major stressor in our relationship was this quarterlife crisis (as they call it). I too couldn't bring myself to just say how I was feeling, how it was making me behave in regards to my future, how it caused me to start acting out by doing stupid shit with my friends instead of focusing on what I needed to and left me in the perpetually immature state for a while. Had I done so, maybe there would have been some better understanding between the two of us.

So, my advice to you is let him cool off a bit, then have a frank discussion. Tell him how you're feeling, how you feel out of control and depressed. If he's a good dude he'll understand, give you a shoulder to cry on, and be there to support you and help you get to where you need to be mentally. If he can't do that, then this is doomed anyway. Because it will never work out with somebody who can't be loving and supportive and give you a hand up when you need it.