Can I use 我的上一辈子 when talking about an earlier part of my life which I almost don't recognize as mine anymore?
in your situation, try the idiom “恍如隔世”.
http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000086679
some would write it as “仿如隔世” 
here’s an example of it’s usage:
https://www.thestandard.com.hk/images/characters/20170222230555idoms.pdf
have fun :)