usually when a girl is drifting in a relationship, the guy feels the need to give them a lot of attention, which is the wrong thing to do, because women like to have a chase ... be firm with her and tell her look, its me or him, if she gives you a wishy washy thing like "i need space" or "i dont know what i want", put on a poker face as if you are not too upset and leave, (or kick her out if you own the place, you get what im saying) now of course you dont fully leave, but you have to make it look like that you wont tolerate bs like this, b/c if you allow this behavior and support it (trying to treat her extra nice for her straying, buying flowers to try and win her over etc.) then she will just walk all over you, put your foot down and squash it before you get really really hurt
because if you walk out its a win win, if she really loves you (28 yrs put in would suggest that she does) then she will come crawling back in no time, if she runs off with her "friend" then it was bound to happen anyway and it proves that she wasnt a good women anyway
trust biz markie when it comes to situations like this man, and good luck