Is all really fair in love and war?

Blah. Is it fair to allow someone to become your friend if you are depressive? Everyone has had one of those "dooms day" friends. You know, the one that is always sad about something, no matter how good things are going, and they always think its the end of the world. Being a friend, you support their "sharing" at first... but then it really begins to get on your nerves, and you want to just yell at the person and tell them to get over it. Most of the time, people tend to drift away from the doomsday person after a while, because it can be depressing just to be around them.
I don't want to be that dooms day person. So, I try to avoid talking about anything that bothers me with people who might be my friends. Which then leads them to think that I don't trust them. But, I don't want to tell them whats going on, and then I'd get into the habit of talking to them about it everytime... and then they would feel depressed nearly every time they talk to me.. and that would chase them off. So instead, I don't talk to them about it. Is it fair for me even to try and make friends with anyone if I am sad? I mean, it might rub off on them, if I get really into a funk, then they will feel sad, and if I go over the edge.. then they have to deal with everything that goes with that. I know this really makes me sound completely crazy (then again, I dont hear anyone arguing with that fact even when I'm not really going over the top), but, I wish I could just painlessly cut all my ties. With everyone. Without putting anyone out. Without making anyone sad. Without anyone going.. "Oh where did she go?", or "How did I get so out of touch", or anything like that. If I have no connections on earth, then I don't have to pretend to be ok. I don't have to worry about what will happen to so & so if I go over the edge... "blow out my candle" as I saw it once in a book. I realize its definatly a step down on the spiral to be having these thoughts.. especially regularly. I didn't think I'd get these thoughts back. It was all such a comfort a little over a year ago... now I'm back to fighting it a bit... I don't know what to do. Go get help is the logical answer, but thats not possible. Its just not going to happen. Theres nothing to be so down about...life is just the same as ever... Theres just nothing to look forward to any more.