Start learning about meditation, but like, for real. I went through the same "mid life crisis" (I'm 23) and it helped me immensely. Especially vipassana. I'm still a massive massive newbie but I can get a tiny tiny grasp of what true happiness is. Startups are an amazing work environment but they're also incredibly stressful and sometimes just shade away what's the real meaning of life. Building startups, optimizing UI/UX, etc etc etc is NOT real life. Just my 2 cents