While she may have been the only one that you have fully opened yourself up to so far, she is not the only person that you can open up to. It takes time to really get to know somebody before you want to, and I'm sure it's a daunting task seeing as how you just invested 3 years into your last girlfriend. Hopefully with the experience you gained from this relationship, you will better be able to not only choose a suitable mate for you, but also be a better boyfriend then you were before.
And you do need some work. I think you assumed you were going to get back together like you did the first couple times and it probably really shocked you when you realized that you couldn't anymore. A break is exactly what it is: the ending of the relationship. If it means something to you, you should fight to keep it so it doesn't happen again. To really learn from your mistakes. Even after that, there is no guarentee that they will be there. That kind of comfortable is not good, because the relationship isn't growing anymore because you aren't putting the effort in. The work has to be consistent in order to grow together. You aren't to blame for all of your problems, but you are responsible for how you handle situations and what actions are taken on your end.
There is no magical way to take away that pain and there will always be a void until you are with somebody new. Until that, all you can do is go to the gym, hang out with your friends and all that. It only brings temporary reprieve, so you have to focus and do these kinds of things every day, and that will help pass the time. If you aren't ready for a new relationship, I understand and I'm sure you will change your tune when you meet the right girl. Until then, have fun, date casually, and so on.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.