In terms of receiving affection, I should be effectively considered an orphan; no tiny violins please, thx.
The first and by far most important thing you can do is to form a few close friendships. Find some guys (assuming you are a guy) that you can hang out with, and spend of your time doing so. This is something I see almost every guy in your position has failed at. Instead of forming close lifelong friendships they put all their social effort into a romantic relationship. Then when the relationship ends they have almost no social circle to give them emotional support.
How the hell does someone in my position find happiness without a significant other? I really could use some quick help because I'm very, very lost right now and my mental health is failing me quite quickly. I'm starting to become a bit worried.
Here are some thing you need to do:
1.
Go to the movies, go out to eat, go out alone and do the things you two used to do together. Part of your problem is that you are used to doing those things with you ex, and now when you do them alone if feels like something is missing, and it leaves you feeling unfulfilled. This is simple conditioned response, exactly the same as an addiction, and the best way to break it is to do those things with out someone. It will take a few months, but eventually you will start to feel normal again.
2.
Spend less time in your house. You are currently probably spending most of your time just going between work and home, partially because of number 1 and partially because you don't have a strong social network. This will cause anyone to be depressed. You need some excitement in your life. In the past that was mostly provided by your ex. At first because of the sex and NRE (New Relationship Energy, that feeling you get when you are with someone you really like) and then later because of all the fights and tension in the relationship. Now you need to generate your own excitement. You need to find things to do that interest you, that give you something you are excited to talk about.
3.
Work on yourself. Fix some insecurities. If that means hitting the gym to bulk up some, that is great. But it might also mean getting Lasik and ditching the glasses, or getting braces to fix crooked teeth. Look into yourself and find what makes you feel insecure and start working on it.
4.
Introspection This is the hardest thing I'm going to ask you to do. Spend some time with yourself and ask the hard questions and answer them honestly. Then once you have your answers, do something with it. Which leads us to the final item...
5.
Take some risks! This is probably the best time in your life to take some real risks. You probably have very little to lose right now, and a whole lot you could gain. You probably have no one to answer to but yourself. So, have you dreamed about becoming an entrepreneur? Getting a Ph.D? Backpacking across Europe? Go for it! Even if you fail you are going to have a hell of a time doing it. And in the end that is what it is all about.