I've been feeling really down in the dumps lately, because I feel like I missed out on life
I'm sorry to hear that. Does your feeling of having missed out on life have further implications?
Yes, "I miss opportunities in my past, I feel sad about it but I'm not going to change anything anymore."
It's unfortunate you missed out on some opportunities in the past, but you should not obsess over the past. Instead of worrying about the past focus on the present and the future.
"All I have is the present and I have to make the best out of it, nothing else matters and I'm not going to waste all my precious time."
Would you really being wasting time by trying to experience the great things in life? I'm sure you have plenty of time life to get out there and enjoy life and experience successes.