Why am I always left out?

I have been consistently overlooked or excluded my entire life (I'm 26 now). People think I have a lot of friends, but I feel like I have to chase all of those friends down. I've even told people how left out I feel and asked them to let me know the next time something's going on. They all say they're so sorry I feel that way and it's not fair to me, but they still don't invite me the next time. So I try again and the same thing happens. I try to throw my own parties and invite people out for coffee, etc., but again, I feel like I have to chase people down. People say that I'm fun and nice and pretty, so I don't know the problem! Please help? I hate feeling so undesirable, unwanted and unaccepted my entire life. It feels good to be accepted!