"We tried to make our lives work together, and we realized they don't," said Lamas in Us Weekly's August 11 issue.
"Matt and I's relationship was on blast from the beginning, and that's one thing that ripped us apart. I'm used to the Hollywood BS, but it really got into Matt's head."
Lamas' passion for partying was evident almost immediately after she got engaged to Grant, as her appearance on the cover of Girls Gone Wild's June 3 issue was made public the day after he the popped the question during The Bachelor: London Calling's pre-taped finale in May.
While Grant initially characterized the provocative photo spread as a "storm in a teacup," it apparently struck a nerve with the British The Bachelor star.
"After he moved to L.A. and saw her constantly partying and clubbing, he realized that she wasn't behaving like a woman who was ready to be in a serious relationship," a source close to Grant told Us.
"Shayne seemed more interested in getting publicity and being part of the Hollywood scene than she was in the relationship."