If you can't get a nice chair it seems like many people these days resort to using a standing desk but i never tried it. I found this post about it, it might help you make a decision:
http://smarterware.org/7102/how-and-why-i-switched-to-a-standing-desk
I had a hard time finding a nice setup since I'm really tall (198cm), but after spending some money I got something that is comfortable enough for me to work. I think that besides the chair the height and distance at which your monitor sit is also very important. This is what you can do, just sit straight (back, shoulders and neck all straight) in your chair, position yourself at a comfortable reading distance from the screen (this varies according to how bad your vision is, but it should always be more than 30cm) and look forward with your eyes relaxed (don't force them) you will notice that your eyes naturally position them selfs in a small downward angle. That point that you're looking at is where your monitor should be which I'm sure is much higher that where it's at right now.
About my depression, I don't want to get into too much details but the turning point for me was when my teeth started to break because i was always grinding them against each other. I would often wake up with small pieces of teeth in my mouth and i started to get scared. A friend forced me to see a doctor, I toke anti depressants for a month (be careful with anti depressants or anti anxiety pills, they are usually highly addictive) and talked with a psychiatrist for a few months an that really helped me.
I think that what usually happens is that people don't realize that things will get better as they grow older and they start loosing hope. Something bad happens and then in your mind you start to extrapolate that and begin convincing yourself that it will always be like that. Well, it wont. I think this article explains it much better than me:
http://www.cracked.com/blog/5-reasons-life-actually-does-get-better/





