dex, I am in a very similar situation with my friends. They all live fairly close to each other, and I live on the other side of the city, not really far away, but far enough that I can't always join in on spur of the moment activities. And it just seems that if you miss one thing, then people are less likely to invite you to the next thing.
There also seems to be another dynamic with my friends...maybe it applies to yours, too. For some reason, even when I'm in the midst of the worst depression, my friends have this notion that I am ok. I'm the "strong one", and nothing about the reality of my situation seems to shake that belief. I once told two of my girlfriends that I had terrible panic attacks about driving into the city to meet them that evening, and they listened and tried to understand, but I could see that my panicking over something like that was too far out of their concept of me for them to really accept it. From what I see of you here, you also appear to be one of those "I'm ok, I've got everything together" kinds of people, so perhaps your friends can't believe you really need them, no matter how much you tell them you do.
I have also noticed the same thing here at work...people assume I don't want to be asked about lunch plans and things, even if I try to tell them I do. I have no idea what I do differently than others to project this "I don't need any help" aura...but I have to believe that I am doing something, since this pattern repeats itself over and over again, with many different groups of people.
Now for the good news...I have managed to change this pattern of being left out. Even though it really felt like desperation for awhile, I forced myself to be the one to ask people what they were doing, and even to invite myself along to things. I will even plan activities, so that I put myself in the position of asking people to join me, instead of be asked to join others. I know how hard this is to do when you're feeling depressed - there was quite awhile when I was not strong enough to manage it - and maybe you're not at a place where you can reach out so aggressively yet. But I can tell that you are getting stronger, and if you can get yourself out to do things even now and then, people will start to remember to ask you along for more things. It's not perfect for me...there's still times when I'm home alone because my friends just forget to call their friend who's an hour away...but it is so much better than it was. And when I really need to do something, I know that I can call and ask them if they want to get together.
Just one more thing, before I wrap up my novella.

I do think it's easier for people to cancel out on a visit to someone's house vs. some kind of activity. It doesn't sound very considerate, since your friends should know that you need people to visit, but it's the kind of thing that people figure they can always do another night. Even though you don't want to make these visits special (maybe because you are afraid of the disappointment if "special" plans fall through), anything that can make it appear that there's a reason you want the person to visit on that particular day...making dinner just for them, renting a movie that will have to be returned...might help encourage them to find a way to make it instead of letting a little thing cancel it out. And of course, if you can possibly make plans out of the house, going to a movie or dinner or something, would probably make it even more likely they will actually make it.
Good luck. Like I said, I can totally relate to what you are going through. And it's not easy, but you really can change things. Because I know you are a lot of fun, and a really great guy, and these friend definitely want to spend time with you.
*hugs*
Jo
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If she spins fast enough then maybe the broken pieces of her heart will stay together, but even a gyroscope can't spin forever</font color=green>