I'm not a therapist or a guru, but an arrogant 28 year old who has a B.A. in Psychology and studies Zen. Here's my advice:

Life will always be %#@&#! at some point or another. You might think "if only I had married him when I had the chance" but do you really know if that would have solved everything? You still would have fights and problems, and you might have ended up in the same position of loneliness. A marriage would not be a guarantee of marital SUCCESS. You get me? A marriage is a risk, not a sure thing. What you passed up was a possibility of something great, not something great. It's only been built into something great IN YOUR MIND, based on what you've seen of him afterwards. But you don't know if you would have been happy with him, or he happy with you. You don't even really know if he's happy now, do you?

On the contrary, you broke up with him. Maybe it was for a good reason. Maybe you had an intuitive leap and figured out that it wasn't right or that you weren't ready. Maybe you avoided some great disaster. Maybe you knew that you weren't ready to get married, and based on your post, it sounds like perhaps you still have some more personal work to do before making a commitment like that.

To get you out of this, I think you have to assess what's making you unhappy. You think it's not having someone around, but I think it's your attitude about not having someone around. Many people are alone and don't feel lonely. They're fine with it. You, instead, obsess over it. So the answer is not to feed you your obsession, but to unravel it. (If you had a guy right now, I think you would probably hold onto him too tightly anyway and run into problems.)

So, how do we do that? Start by imagining yourself in a better position, but without a boyfriend or husband. Can you see yourself a bit more at peace? I can. I can see (not literally, but theoretically) you relaxing and doing your own thing, still dating, but not making it the end of the world, where your quest for companionship is injected with humor and levity and you gain your happiness primarily from the activities you can accomplish ALONE and the stuff you already have that is INDEPENDENT of someone else. The "someone else" is just icing on the cake. The "someone else" is not the cake itself.

Getting to that place is tricky, but not impossible by any means.

1. Understand what the problem is. As long as you think your problem is not having a guy, your ability to resolve this thing will be impeded. If you can see that your problem is your attitude about being alone, you'll be in much better shape, and, it's easier to fix an attitude.
2. Visualize your new self, independent of any guy, or still search for a guy, but not making yourself crazy about it, and even having a smile on your face while you do it. When things get a little crazy, laugh at it!
3. Be patient with yourself and the process of overcoming your depression (I guess). I prefer to think of this as tripping yourself or getting in your own way rather than depression.
4. Start doing things that refresh your sense of authenticity and sense of self, and stop doing things that reinforce your dependency on others. Use wisdom in deciding what those activities are. Use your feelings as a measuring stick. If something makes you feel calm, independent, satisfied, and like yourself, then enjoy it and increase it. If something makes you feel like *****, like going through old photos, then tell yourself "no," gently but firmly. If it's a certain thought pattern that you keep playing in your head, then notice it and laugh at it, saying something like: "oh snap, here it goes again!" or whatever makes you smile, and just watch how it tries to work its magic on your mood and thoughts. The more you practice, the weaker its grip will be. Creative activities work wonders in refreshing your authentic self; so does helping other people. Collecting things, writing, art, dance: all good options. Stay away from drugs/alcohol, though.

You may have to force yourself to be happy, which is a weird thought. Lots of people hang onto their depression because they are comfortable in it and identify with it. If you find that that's you, then SNAP yourself out! The best way is to just force yourself to say "F*@k it" and be okay with where things are. Being silly is a fantastic way to do that.

5. After you've started to refresh your sense of self and snap out of this thinking, perform some kind of ceremony. It doesn't matter what it is, and the more personal you make it, the better. The purpose is to shed your old self and step into a new self. It doesn't mean you'll be problem free, but it's a sign of your commitment to address your problems differently, with humor and with dispassion, rather than going f'ing nuts. The ceremony can be anything. Use creativity to take your pain and make it into something positive. I took old pics of my girlfriend and I and that I saw as me being fake and needy and turned them into a collage. That was my ceremony. Maybe you have some old pics or clothes or emails you can do something creative with.
6. Keep practicing, and keep your goal in mind: a person who is alone, but is still okay, still living, and still enjoying their life. Occasional thoughts of guilt or anger or frustration still arise, but they're addressed (through humor, or through self-care) instead of self-criticism and panic. It's very very very possible, especially after you receive fantastic advice like this.

Finally, I'll close in saying that you already know all this. It's just a matter of digging through the pain, waking up, and exploring something comfortable inside you. It's all there already.