I feel you. I'm going through the same right now. And I hate to say this but I think that you can only fill this void by making peace with it. I'll explain my point of view:
TL;DR: This void is like an abstinence and no matter how many things you do, you should give yourself time to overcome the addiction and be fine again. But in the meantime don't just wait for things to be better. Work on yourself and your life and be patient. Things will be ok eventually.
I've been always sticking to my plans and investing on myself. I workout regularly, I work hard on my studies, I invest a lot of time on my hobbies - namely, music and arts. And I've been doing all of it ever since much before the relationship that made me need to join this group. So... What I'm saying is that, like I'm still struggling despite of doing a lot of stuff, no matter how many things you do the void will still be there for a while. Because it is just natural as you brain overcomes the addiction [1][2] to the person and the relationship. This void is basically abstinence. When I say that you should make peace with it what I mean is that you should give yourself time - and this amount of time is totally personal - to heal and no longer be an addict to this love.
But it's not all sitting and waiting. I completely agree to the others here suggesting that you should put yourself out there, hit the gym, work on your hobbies and career etc. This will not exactly fill the void, but will make faster, easier and (more) pleasant the process of making peace with it and solving this problem. Besides, doing that may take you further much beyond overcoming this relationship. And, like one of our fellows have already stated, this is the best time for you to be a better you. Please don't waste all of this "introspective energy" you have inside of you right now. Hard times really make us better. I recommend at least working out, because it releases neurotransmitters in your brain that will ease your pain, besides contributing hugely for your self-esteem, what also helps a lot and will leave lasting benefits. Seriously, I can tell you by my own experience that working out has saved me uncountable days already. Some days I wake up feeling like crap, but after working out, running or bicycling I feel like a hero.
Best of luck. :)