I've struggled with the exact same question. I realized I needed an obvious purpose, but that's secondary.
I "press on" because that's life. What I mean by that is that, although you're bound to end up with a loss on the scale of happiness vs. suffering, being alive and going on with it is the only way to ever experience happiness. Life isn't fair, but not being alive leaves you without anything prematurely. I've once made a comparison to being on a deserted island where there is one coffee plant (the good stuff) on the top of a mountain in the middle of a freezing lake. The things you would go through to achieve the end-product of one steaming-hot coconut shell of freshly made coffee, are disproportional to the actual gain. But putting life on a scale like that is bound to drag you into a depression - like I said: you're going to end up with a loss.
Secondarily, I figured we, as a species, are part of a common struggle with that balance of happiness and suffering. Basically I regard it as my main purpose to sustain life and, most of all, try to decrease suffering and increase happiness for the people around me. That notion is what keeps me going most of the time, since it is hard to ignore the imbalance of the previous alinea. Altruism is sometimes a weak motivator, especially in the face of overwhelming evil making you wonder why one bothers to help other people.
So you keep going because that is the only way you'll come across happiness and, while on your own way, you can make an effort to help others towards happiness.
I think it is not such a personal position, basically I trust all healthy people see their purpose, either philosophically or just driven by natural instinct, to care for others. Of course, this entire wall of text falls apart when you believe in an afterlife... The being temporal of life and this world is essential.
