Lost...

My "fiance" and I have been together for 6 years... I love him and I know he loves me but it's like history is repeating itself, I feel like I did in highschool, like he acts different in front of his "new" friends. This is not something I feel adults should do nor go through, especially someone who hasn't acted like this in years until this past month... I feel like I shouldn't give up but at the same I already dealt with my share of this behavior when we were kids. I just don't know what to do next, we live together :(