I cheated on a partner (with an ex, just to make things that much worse).

I’d say it was caused half by lust, half by my insecurities at the time. I wouldn’t do it again, as it was detrimental to my partner, my ex, and myself. I also like to think I’m slightly less insecure now. I haven’t been in a relationship since that one though, as I still feel I probably have a bit of maturing / sorting myself out to accomplish before being to handle a healthy romantic relationship.