I was struggling to get into "con-mode" myself up until a month or so ago. For me it's almost an uncontrollable natural mood. I can't make myself get excited about the con. It has to just happen on its own, and I was pretty worried that it wouldn't happen this year, but once I jump-started "con-mode" by watching a lot of anime and researching cosplay, looking at cosplay pics, my excitement eventually picked up. Now I'm stoked and can't wait. ^_^
But I definitely know the hollow, unsatisfactory feeling of knowing con is coming up and that you should be excited, but for whatever reason not being. That really stinks about your friends. I know such a big part of Kumoricon is friends and the relationships that allow you to cut loose, but maybe this can be an opportunity for you to make some new connections? I'd encourage you to try to stay active on the forums and connect to as many people as possible. Seeing new friends can be a great way to make Kumoricon an exciting anticipated event (just as much as seeing old ones). And regardless of who you end up going with, I'd encourage you to not go through the con weekend passively (just staying in the hotel room like I've felt like doing when not interested/motivated), but still just get out there. Just being in the con-space can do wonders for excitement and con-mode.
As far as other things to try to jump-start your excitement, immerse yourself in all things otaku, heh. Watch anime (even if you don't
really want to...just put it on and let the "consmosis" take effect, haha), maybe visit Uwajimaya with your boyfriend, pick up some iconic snacks like pocky and ramune, watch
these videos immediately, and every other day until Kumoricon. They were the magic trick for me really getting excited about Kumo.
The cosplay aspect is unfortunate as that is another thing that really keeps the blood pumping, so to speak, for Kumoricon. Even if you don't have an elaborate costume, I'd still encourage you to look up other costumes. Browse Cosplay.com and see what others are doing and let them inspire you for future years. You may have finished a more simple costume for this year, but you can still plan other costumes you'd like to do for other cons/years. I've found that to be pretty helpful this year. I have plenty of work to keep my busy (too much perhaps, lol), but for the down times when I have nothing to work on, browsing Cosplay.com is a great way of keeping me motivated and interested.
All in all, the lack of excitement sucks, and I'm sorry you're going through it, but I think if you stick around the forums, you should be able to build up that passion again. ^_^