Callum ditched Shaughna because she's not 'attractive enough'.
As great as I think she is, her face is distorted with fillers and she has big legs. Callum wants a 10/10, a 'Molly' type, he's a shallow young lad that will banter about 'worldies' in twatty pretentious bars! He's not the type to accept someone for their inner beauty, imperfections and all.
As someone said this isn't her first rodeo, unless she stops going for the 'high school jock' type. She will always be let down, unfortunately she isn't the 'prettiest cheer leader in camp' and that's what the 'jocks' want.
I love Shaughna, she's a beautiful intelligent woman - she needs to change her 'type' though otherwise this will just become a pattern.