I need girl advice.

My girlfriend just dumped me because I was spending alot of time with my family and my best friend that's a girl cause my uncle just died, and my uncle liked my friend like a daughter. I wasn't spending enought time with her and I was cheating, or so she said. Is it just me or is that seriously fucked up? Should I try to get her back or just let her go? I mean she's really nice and I like her alot, but she's not the brightest bulb on the Christmas tree. I need help.