My Dear Guest, How did your partner come to meet your old girlfriend? More importantly, how many of your old girlfriends have you hauled around in her presence? Perhaps she is trying to make the point that she would prefer not to be given a list of your conquests. Perhaps too, she is tired of being nagged about the Hoover and the cooker and is taking refuge in the company of a sympathetic friend.
Why is she forced to hang around charity shops? Is she not given enough money to buy something new? Sounds to me like you are looking to have your cake (great in bed) and eat it too - cheap labour.
Two pieces of advice if you wish to salvage the relationship: 1) DO NOT allow this other female to accompany you on holiday. You need time alone together to communicate, clear the air, and start afresh. 2) Do not pile her and all your belongings in an RV or van and drag her around to folk festivals and basement accomodations in friends' homes. Part with a few dollars you cheap SOB and let her enjoy a first class hotel complete with room service and a masseuse. Charity shops...good lord!