Okay Bees, I need advice!

I have this friend, M, who I used to be best friends with. We were roommates in college, had a lot of the same classes, did homework together, etc. After we graduated, we both got similar jobs at different companies, but in the same city, so we ended up living fairly close. We were each other's Maids of Honor, and our husbands are close as well. We used to get together EVERY weekend.

Fast forward to now. Now we're not close at all anymore. I'm not entirely sure why... but I don't feel like we have much in common anymore, even though we probably still have all the same things in common that we used to have. It just feels like our friendship has stalled, and a good part of it is my fault.

About a year ago (a little more), M called me out in front of a group of people because my husband and I weren't going to a party they were throwing. She basically threw a fit and insinuated that I was a bad friend for not going, even though I had gone to this annual party every year for ~6 years and this was the FIRST time I was missing it. Ever since then, I've had trouble getting together with her... I guess I'm still bitter?

Anyway, ever since then, we really haven't seen each other much, and I even tend to screen her calls. She has the tendency (at least in the past) to try to make me feel guilty whenever I have made plans without her (even though I have other friends and she has other friends she does stuff without me).

So, anyway... she e-mailed me today and basically called me out on it. She asked if there was something wrong with us and that she feels like she never sees me anymore (mostly true) and that I don't call her or answer her calls (pretty true) and that she is always the one trying to make plans with me (not entirely true). So now I'm not sure what to do...

Should I tell her that yes, there is something wrong with us? I'm worried about creating even more tension and losing her friendship forever. But on the other hand, I'm not sure if I even want to put in the energy to fix the relationship... help?