Am I overreacting?

What got me was, "I forgot you were here." I'm not even in your relationship and that stung me a bit.

My suggestion, have one serious talk with him. Try your best to not cry or get angry during the talk, just listen; imagine you're a private investigator trying to find the truth. If he refuses to talk to you, or continues to tell you he's fine, then tell him, "You've been treating me like shit lately. If this is how you are when you're fine, then I guess I'm fine too," and just do your own thing. Don't cater to his moodiness, and just enjoy life.

Right now, he's going through something he thinks you're not worthy of knowing. If he wants to handle it alone, then let him. Life is too short to be caged up by someone else's emotions, and life is too short to be forgotten by someone who claims to "love" you. If you're not ready to end the relationship, at least go hang out with your friends more and let him just be an asshole to himself.