It sounds like you're going through a grieving process in response to the loss of a person who meant a great deal to you and with whom you spent a significant amount of time. It's hard to get over such a loss. If you think it'd be a good idea to get back out into the dating scene but are unable to do so, we first have to figure out what's preventing you from taking that step.
If you're still simply grieving the loss of the relationship, I would say give yourself a little more time. I think it can be counterproductive to force yourself if your heart's not yet in it because that's likely to interfere with your dating process. Let's face it, no one wants to go out on a date with someone who talks non-stop about their ex, or with someone whose heart and mind are otherwise engaged.
On the other hand, sometimes thinking obsessively about an ex can be a way to protect you from making yourself vulnerable again. It's easier to dwell on the past and to romanticize what we had than it is to take a risk and open ourselves up to someone new and all the messiness and imperfection-and, hopefully, all the joy and excitement-that comes with that process. If this is the case, my advice is to be gentle and compassionate with yourself and your heart. You've been through a loss and have experienced pain, so you're understandably fearful of experiencing the vulnerability inherent in a romantic relationship. I encourage you, however, to test the waters again at some point in the near future, and to do so such a way that honors your still tender heart.
You might want to start with whatever feels the least risky and emotionally demanding. Perhaps you simply go to a party the next time you're invited. Maybe bring a friend along for moral support and give yourself permission to leave early if it just feels like too much. Remember, this isn't a race and there's no need to get married tomorrow. Try to have fun; let your attitude be that you just want to get to know people and expand your circle of friends and acquaintances. And if you find you click with someone, just remind yourself that you're going to take it slow and remember that you can be the one who determines the pace at which things unfold.
I think that once you can trust that you have some control over the process itself, you may not feel as scared and vulnerable, and this may allow more room for you to feel willing to move forward.