I spent a while in a state that I guess must've been depression. I was totally uninspired, it was a struggle to even get myself out of bed in the mornings, I didn't want to do anything at all. It wasn't like crying uncontrollably, it was much more just like wanting to curl up into a ball and staying there until something changed. And I've no idea what caused it or what fixed it, but I did eventually get out of it (more or less, I still have the occasional recurring lapse that lasts a few days).
A girl I went to school with ended up in front of a train a year or so after we left school. I didn't really know her, but I was still affected by hearing what happened. At first they weren't sure what happened, if it was someone pushed her, if it was a drunken stumble, but eventually the conclusion was she jumped. I can't even start to imagine what it's like to lose a friend like that, let alone multiples. Of course my guess would be the first one had issues, then the others after that just couldn't deal with it.
I guess you feel guilty because you feel like you're the link between them all, therefor you think you're the reason they did it. Obviously that's just not true. What the link might actually be I don't know (because I didn't know them and I don't know you), but I can tell you for a fact that it wasn't you.
As for you doing something about it? You already know you couldn't have stopped it, you just
wish you could. People who want to end their lives know exactly how to do it in such a way that nobody can prevent it. The people who 'attempt' it and fail are just looking for sympathy (or in some cases attention). Suicide may solve your problem (it might not, we don't actually know if what comes after is any better), but it creates a whole load of new ones for the person who finds you, as well as your own friends and family. You know what it's like to be on the other side of this situation, and hopefully you won't want to put someone else in the position you're in.
Now, advice. That's a hard one. Start simple. Don't try to change your entire life in one go. Do one single thing today that you wouldn't normally. Exercise is an easy one. You probably don't want to, but it's the easiest, quickest and cheapest way to get a shot of endorphins. Sit-ups or push-ups don't require you to move outside the room. Jogging is another, but obviously needs a bit of space. If you don't want to be around people it might not be a good idea.
Do any exercise to music. This will help to create a mental anchor with that music, meaning you will associate those tracks with the endorphins and after a while you might just need the music to help you. Obviously try to avoid sad songs or any that might stir an emotional downer - you don't need that right now. This might be a bit harder if you're into ballads or are a bit of an emo
But try. Even if you hate a song, it'll be worth it in the long run.
Try to avoid general news. Sounds weird, but you don't need anything that could make you more miserable. If the world does end, I'm sure you'll find out about it from someone else. If on the rare occasion that something good does happen, try to read as much about that as possible, it might help to convince you that good stuff does actually happen.
You have an unusual job, I assume because it's something you're interested in. If so, appreciate the fact that you have a job you like. The majority of us don't. In this respect you should thank your lucky stars that you're not
If you
don't like your job, try to start looking for something else. This could be another source of your depression, and while it's not something you can actually fix 100% by yourself, the beginnings of change lie in your hands. There are hundreds of websites (I can recommend a couple if you like) you can set up profiles to be informed of new jobs without leaving your house and having to sit in a job centre (which are depressing places already - trust me, I know). The advantage of using websites to look for work is that you can actually do it from your current job, so you don't need to use up holidays or unpaid leave. In a depressed state you don't need to speak to anyone and it doesn't take any real effort to do.
This is turning into a bit of a book, but I think I'm done. That's all I can suggest without having any questions to work with. If you want more specific help, PM me here and I can be emailed or contacted via MSN.