What keeps me going is my marriage and children. I have very problematic parents (homelessness, drug abuse, divorce). I also have multiple siblings. I have zero support or socializing from any of them.
Having people that rely on me for their well-being and knowing I can provide them a better life than I had is sustaining in the tough moments of dealing with an existential crisis.
It also helps that I strive for mastery of my craft. I’m a software architect and there is lots to learn. Even if it’s true that nothing matters, you have one life. Make it as best as you can. Very few humans can claim mastery in a skill.