At the moment your relationship is being conducted from a distance... if you can't talk to her online or whatever then what is left? I'm not against supporting partners through difficult times but a relationship is supposed to be mutual and that often gets lost whilst one partner tries to fix (or cope with) the issues the other has. What are you getting from the relationship? Your needs are just as important as hers.
If it is indeed a seasonal thing, and you want to work at this despite the issues then maybe back off more and see if she comes around.
Depression is serious. And the fact that she has not just contemplated suicide but has talked about is very unsettling. I know you can’t support her as much from a distance, but she should be getting some help. I don’t suffer seasonal depression, but I have gone through my bouts that escalated in the last year. It completely changes who you are, what you respond to, what you want to do and who you want to do it with. You feel very disconnected. It takes a lot of work to get out. Unfortunately, I couldn’t maintain my 3 year long relationship because of what I went through last year. I imagine it will be even more difficult for you to do so from a far. I think you should focus more on convincing her to get help than just focusing on the relationship. Once she gets through the dark cloud, she might come to sorts and realize you hung around and supported her major issue versus focusing on just your own happiness. I wish you the best of luck. It is difficult but you must ask yourself if you’re willing to go through the effort.
Love For Me, Not For What I Can Do For You.