1. Go out to the bars and clubs, drink as much as you want, meet women, have sex with them.
2. Find a hobby, something you enjoy and stick with it.
3. Venture out on your own and start a business to your liking.
4. Quit your current job and find something new, even if its almost the same, maybe a new environment will make you feel better.
5. Move to a new place, even if you still have the same job maybe moving might make you feel like your getting a fresh start.
6. If you have no girlfriend, find one, that might take your mind off of things.
7. Buy something, a new computer, a game console, a car. Anything, to invest time into.
8. Find a job in a new field.
What it mostly seems like, is that the repetition of life is boring you and making you feel depressed. It can be hard to look ahead and feel like you will be doing the exact same thing every day for the rest of your life, its scary and quite frankly it is a little depressing. That is unless you enjoy your job, if you don't enjoy your job then its almost a burden to wake up and go to work everyday, especially when its all your life revolves around.
Meet some friends and hang out every now and again, take a walk down the street, go out to a bar if that's your thing. Socialize, being alone and having no one to talk to is actually destructive especially growing up in a society that depends on us being social. I know the feeling your going through, but spending time with people is actually a way to get your mind off of the current situation, at least momentarily.
So work is feeling like a never ending loop of the exact same thing day in and day out. Spice things up, if your current job and environment is whats making you feel depressed, then search for a new job in the same field or different field at a different establishment. That might freshen things up for a while, or maybe work towards a goal? Say you want to purchase a house, save and work towards that and use work as a template for getting your self closer to that goal.
Realistically, I feel that you might be scared and set off by thinking about doing the same thing day in and out, for the rest of your life. Either this is a true feeling, or a feeling of anxiety. You need to look within, and figure out if this is just anxiety being brought on which is causing these depressing thoughts. Or if this is actual boredom, and your actually tired of working a 9-5 everyday and you actually need something else in your life to take away that feeling. Either way, I wish you the best of luck OP.