I have the bad habit of skipping classes when things get rough, too. :|
Do your professors know about your anxiety? This probably sounds really scary, but it might be a good idea to set up appointments with your teachers at the beginning of each semester and talk to them about having SAD. By telling them that class can be a struggle for you before classes really get started, most of them will be more understanding if probelms arise in the future, where as if you talk to them when the problem comes up (like during midterms or something), they may be less inclined to believe you.
I wonder if there's a way you can get documentation for SAD. Some school health centers might have a program or test that can prove you have a real condition, so that even if a teacher doesn't care about it, they have to take it into account anyway. I've done it for my learning disabilities before, but I don't know if you can do the same thing for SAD or not.
As far as what you can do right now, maybe try emailing your teachers and the extracurricular thing you have? Perhaps try to explain that you've been having some problems, and ask what you can do to make it up?
I hope it gets easier for you. Class can be brutal.