It depends on whether or not you know for certain if you’re the only one alive.
If I was literally the only person still alive I would have to deal with my grief first and foremost, which would be horrendous because everyone I love and care for would be dead, and then with the knowledge that I likely wouldn’t live very long either (unless I really managed to get a handle on survival. Which to be fair I probably would, given the strength of the survival instinct.
In the meantime, whilst I was still alive, I would live every day with the utmost appreciation of the privilege of my position, and with a deep appreciation for the beauty I would still see in the world.
The pain would be immense, almost too much to bear at times, I imagine, but I would resolve to revel in whatever pleasures I could find. Because I would owe it to myself, my loved ones and to the whole of humanity.
What I would practically do with my days would take a little too long to write out now, but it’s an interesting question, so I’ll try to return to it later.