Through my many years as an entrepreneur, one of the hardest things to deal with is the constant flow of thoughts. Your girlfriend were speaking to you, but all you were thinking about was that one support case that were hard to solve. I remember when my mother turned 50, but all I could think about was how I would be able to pay the bills next month.
It's hard not to constantly think about the many important issues, but this constant flow of thoughts is what will drain you from energy in the long run. You need to learn how to stop thinking about your business and be present in the now to build up the energy to solve all the hard problems you face.
Well, it seems counter-intuitive to do something that will make you think even more, but the secret here is that you should do something that will consume your full bandwidth for thought that is not work!
I personally found off-road motorcycling. I am required to be fully present in the now, or I will crash. I can't think of the support case mentioned before when I am in full flow down a narrow single track route.
The practice of forcing you into thinking of something completely else trains your mind to get used to not thinking about work. It takes a bit of time, but you will get used to the fact that your business is doing just fine when you don't think about it and you will only get better at solving hard problems by giving your brain a break from thinking about the same types of concerns.
Give your brain a break, go out and play a sport, build a house or debate politics with your uncle. It will save you!