"We hoped to see you at XXXX but never connected. We've been seeing you off-and-on for the past couple years, which has been really nice, but we have never talked about the long estrangement. I'd like to do that, and to make whatever apologies and peace are still needed. Perhaps we can have coffee sometime. I'm usually flexible on Fridays and the weekends."
That was a surprise. The planned meet-up fell through for very good reasons - I got done with one thing only a couple hours before I really needed to get to bed to make a 5AM flight. However, the relationship probably had gone weird a long time ago. We've been seeing each other once or twice a year, but without ever really talking about why there had been a long period of silence. I personally am a fool and a flake with heart with a good memory: I might not see you for a long time, but I'll typically feel the same way about you. Here, I just felt that there wasn't any real interest: it's hard for me to read people. I try to wear my enthusiasm on my face: you should likely know if I'm enjoying your company.
Pretty much if you want to see me more, make plans with me. Otherwise there tends to be a billion things to do, and I get caught up in them and don't see people I should be or do call friends for far far too long. It's so easy to lose track of everything in this modern era. I'm terrible at it.
So yeah, if I haven't seen you in a while, I'm sorry. I pretty much guarantee I still think you're as awesome as you ever were: I'm just terrible at planning.