Hey Paulos,
I know exactly what you mean, I got sober just before my 25th birthday and I definitely had the "I just wish I could turn back the clock to when I was 19" kind of thing. But you know, the longer you're working the program (and the more you put into it) the less you will regret the past. I'm at almost 7 months sober right now, and I'm not going to lie, sometimes I still do traverse memory lane. Most of the time, it's pretty much involuntary. But you have to realize that it's not condusive to your sanity/recovery to beat yourself up about these things. Everyone else who's posted on this is right though, you've saved yourself on another 10-20 years of misery (which I'm sure would have brought some new residents to memory lane...if you know what I mean).
For me, it was REALLY difficult not to beat myself up every day in the first two months of sobriety because my job allowed me to just think and think and think all day (I do data entry assignments... so yeah, mindless work...) and I just wasted so much time beating myself up about the people in my life that I had pushed away because of my addiction. People I actually did care about, but I got sick of when they didn't give me what I wanted or refused to see things my way. I still have to make amends to some of them.
What I suggest is when those thoughts pay you a visit (or vice versa) try asking your higher power to remove them from you. And if you have a lot of free time, find a hobby, something to give you purpose. (I'm going to have to give my sponsor credit for that one...)
Thanks,