Purpose of life?

43/m here, straight white man who had all the privilege and supposedly played the game of life on easy mode. In B4 "boo hoo poor white man"

I'm middle aged, and there seems to have been no purpose to it all. I have no biological kids, so evolution has chosen me as a loser. I raised my girlfriend's son from when he was 5 until he was 18. He gets married in a few months and I was not even invited to the wedding- no fight, no major disagreements - just not important to him. He's mildly autistic, so the fact is his dad is his biological dad who he saw four times a year growing up. He's going to the wedding, of course. (dad's a good guy- not knocking him)

I have a handful of STEM degrees, and now teach engineering at a college and tutor math at a high school. I took far too long to get them, and, overall, they weren't worth the time to get. My roles are 100% replaceable.

Ultimately, I've had almost zero impact on this world, and have literally no purpose. I was in a bad car accident at 17 and survived uninjured. Since then, I can't help but wonder, "why," as I've amounted to nothing and have no significance whatsoever. Why survive the car accident to blunder through life to where I am now?

No kids- biology and evolution have said I'm worthless. I'd like to say raising my girlfriend's son was something - but it wasn't, and biology wins there, too- I'm less relevant than the personal santa claus that showed up 4x a year. No great contributions to the world - the last resort of those who have no biological impact. 

I'm reminded of a favorite quote from my favorite author, Ayn Rand:

"Gail Wynand raised a gun to his temple. He felt the pressure of a metal ring against his skin—and nothing else. He might have been holding a lead pipe or a  of jewelry; it was just a small circle without significance. "I am going to die," he said aloud — and yawned. He felt no relief, no despair, no fear. The moment of his end would not grant him even the dignity of seriousness. It was an anonymous moment; a few minutes ago, he had held a toothbrush in that hand; now he held a gun with the same casual indifference. One does not die like this, he thought. One must feel a great joy or a healthy terror. One must salute one's own end. Let me feel a spasm of dread and I'll pull the trigger. He felt nothing."

(I'm not going to do anything, I just understand the sentiment expressed by this passage)