People who say long distance relationships don't work are wrong, mostly. It takes a special kind of person to make long distance relationships work, and I'm sure that you can make it work too. Having been in a LDR for 9 months, I know what its like. One thing that is a key to making LDRs work is setting a date in the future where you know you'll be able to go from long distance to being in a normal, close relationship. My boyfriend and I both decided that if we couldn't be close to each other sometime soon, we would end it. We were lucky and were able to be close. So, that is a very important aspect in a serious LDR. If you can't eventually be close to each other, then chances are it won't work out unfortunately. But hopefully you can make it work.
As for the Seasonal disorder, I have no idea about that, so sorry.