Then as time has gone on, I keep thinking about why I haven't been more successful in life etc. I know that giftedness does not equal success, but I can't help thinking that I have wasted a great opportunity. 
Did you find out your own high IQ as an adult and what effect did it have?
I have felt this way, too.  I see it as an existential depression.  Here's a   
good article about giftedness and existential depression.    
The past needs to be processed and lost opportunities need to be acknowledged; but don't get stuck in a fixed mindset.  The present and future are still open for the making.  Just working towards a few challenging, but achievable goals can help you feel more empowered.  Start by thinking about how you can do just a little bit more with the talents and resources you have; for your family, for your community, for yourself.  And make sure to think regularly about the progress you've made.