How can I overcome regret for these decisions?
Although I don't know much about your life, I am going to venture a guess that if you were in this position in the first place, you are bound have marketable skills and are very privileged. Note that I am not doing the "but the kids in Africa" argument but its reverse. You have already won a lottery and can focus on the prize you did win instead of robbing yourself of happiness by looking at even larger jackpots. I am not saying that you are not allowed to be sad, rather just trying to wake you up to the insane bounty you do already have.
Doesn't it fill you with joy to know that you are leading a freer and more comfortable life than 99.999...% of people who ever lived? The quality of your life is already fundamentally equivalent to those wealthy people you torture yourself about. i.e. you can more or less afford to have a life rich in chosen experiences and organize it the way you want. You already have access to all the good things in life other than the sort of megalomania and diminishing return based entertainment that billionaires like to spend their time with.
By definition it will be hard to stop your feelings through rational argumentation. So what I'm saying is to just wake up to the things you have.
Other opportunities will arise so just be on the lookout. You're around the right type of people for where you want to be so make sure you don't stop hanging out with then. As they say, you're just the average of the people you most associate with.
You could reflect on how you decided to not work with them. Do you have a clear framework or methodology that you use to guide your decision on choosing a startup? Does your framework/methodology need to be adjusted? Do you really want to go down the path? What in your eyes is true success?
I hope that helps.