First, I'm friends with you because of you. If highly intellectual things aren't what you're comfortable talking about (and even when nothing's going on with me, I'm not always interested), I'm happy to talk about something else, because you're what's important, not just the complexity of what you say or how quickly you say it.

I second what others have been saying; this sounds like something might be up with health, physical or mental. And in a way, that would be good, because then you can fix it! I know that having something definite makes me and many others feel better, because it's no longer unknown.

When I was, and still sometimes am, depressed or anxious, I can't follow arguments either. Before I got on the medicines I'm on and into therapy regularly, I couldn't read articles in the New York Times and remember the points, or analyze the validity of their arguments. It's gotten better for me, and I'm confident this will get better for you, too.