Dear Jennifer, I don’t think you did anything wrong. You wanted to “talk with him about [his need for alone time] and discuss why I was never allowed to visit his apartment.” which is understandable. Basically you wanted to know where you stand with him, is that right? Well, who wouldn’t want to know that.

Everyone here can only guess what happened. Here is what is for me the most likely scenario based on what you told us:
You friend might be either depressed or have serious self-confidence issues. Maybe he is an introvert. Maybe all three. When he has moodswings he needs to be alone. He doesn’t want you to see him in a bad mood or worse, lash out verbally at you. So during that time he keeps to himself.
He is also ashamed about his flat and his room mate. Maybe everything is in disorder because he just has no energy to clean up. Maybe his roommate runs around in his underwear. Maybe he still lives with his mother.

When you asked why he needs to be alone sometimes and why you can’t visit, a depressed person might have thoughts like “I’m not normal” “I can’t even invite my girlfriend to my flat, what a looser” “I should have done something about my living situation years ago, but I didn’t, I never make any progress” “I can’t tell her I’m depressed, she will look down on me” “this is all too much” “she deserves someone who treats her better, who is not such a burden” => “I want this problem to go away. I should break up with her. I’m sure it is better for her, too”.

I write this so that you understand that this is not about you, it’s about him. Of course I might be totally off, and he is living with his other girlfriend and when he says he wants time alone his other girlfriend wants to go out with him. Only you can say what feels true for you.

What do you think so far?