I'm not one for opening up but I suppose that was my downfall earlier so I might as well share my experience. It is encouraging to see the P&B is capable of maturity...sometimes!
Depression and mental illness is something that can cause a lot of damage in one's life. Sadly I found out the hard way when I finally bothered my arse to see the doctor earlier this year after suffering major symptoms for over a year. It started off with panic attacks and anxiety, I was finding it difficult to socialise in groups unless we were outside in the open, I thought I was just developing some form of claustrophobia but when i started happening all the time that was when alarm bells starting to ring. My depression hit it's worst (I hope!) in September of this year. I went through a horrible break up, quickly became disillusioned with life and I could feel myself drifting further and further away from my friends and family. Thankfully I did seek the help and I am currently going the therapy and I'm on anti depressants. It was essential that I sought the help as it was slowly grabbing a hold of my life. In order to get better I am currently signed off work and have been since October and my state of mind meant that I had to throw university after a mere 30 days. I was gutted but I knew there was no chance I could do it whilst battling severe depression. I don't education is for me now anyway, if there is one thing I have picked up from suffering from depression it can sometimes let you take a step back and assess a lot of things in your life. I have no "career" that I want to pursue (genuinely I don't, I never have and I never will) however I do have aims of where I want to be in the future, a steady full time job is certainly the long term option and the job hunt will probably begin at the start of 2013. To be honest the vast majority of this year has been a write off for me.
During the really bad times I had it convinced that everyone else would be better off without me and other depressive thoughts. These are common for depression, even mild depression. If you have little or no self esteem and you suffer from thoughts like these regularly I would recommend seeking help, even if it's just therapy it will do you the world of good! For some it can be severe problems with self confidence. I also have been living quite an unhealthy diet however it's not because I eat too much, it's the opposite. Thankfully I could be doing with losing some weight so no major physical problems will come as a result but a deteriorating or vastly increasing appetite is something to also look out for. I genuinely went a day and a half without eating anything and when I did it was something small like toast or something. This is not good for any recovery, a healthy body gives your mental illness a better chance.
As for your friends and whatnot I would say you do learn a lot about them when you tell your friends that you are depressed (if you chose to). Sadly for me one of my best friends of many years has not reacted that well and I get the impression he doesn't want to spend time with me or he thinks I'm a mad man or something, I don't care what his motives behind him talking to me are but I'll leave him to it and move on with my life. One of my friends has been particularly supportive and understands what I'm going through and just knows what to do but then again I help them through their bad times as well, I didn't know this person suffered from depression until they told me when they thought something was up with me and I told them what I was going through so you would be surprised how close depression can be.
Can't really say more without repeating what others have said (which I have basically done anyway), fell free to chuck a PM if you want to talk or anything.
I highly recommend Breathing Space by the way, good to see it has already been mentioned in the thread. Especially if you feel you are heading towards a "bad" period of time (it's not like the Samaritans, it's slightly different and offers more therapeutic help and advice for those who are feeling depressed/down, they recommend phoning Samaritans if you are feeling suicidal or if you are going to harm yourself etc). Although it's not available 24/7 its free.
Here is the website (has some good stuff to read); Breathing Space
Phone number 0800 83 85 87 (Available Monday to Thurdays 6pm-2am, Weekend Friday 6pm to Monday 6am)