My fiance's roommate (lesbian couple)

My girlfriend and I just got engaged last week (we are a lesbian couple), and we were discussing about moving in together last night. Amidst the conversation, I jokingly asked her if she had ever lived with an ex before (I always thought I knew the answer but still..)

She was quiet for a moment and suddenly broke down. She cried badly and finally confessed that her roommate of 5 years, who just moved out this September, was actually her ex-girlfriend.

Prior to living together, they had done long-distance relationship for almost 4 years. This ex/roommate was her first love. However, one year after moving in together, they broke up and stopped having sex. My fiance started seeing other people and had a couple of flings, yet still living with the ex under the same roof. Five years later, she met me this May, and she told me that I’m the love of her life.

Since the beginning of our relationship, I have always known that she was living with a roommate who she is very close with like a real family, and I met the roommate twice (she seems like a pretty kind older sister who is 13 years older than us). I also knew that she had one long-term relationship that she would never forget. I just never knew that those two are the same person.

She insisted that they only lived together after breakup for financial reasons (we live in a really expensive city).. and the ex/roommate moved out in September because they both knew that our relationship is getting serious and decided to finally move on.

My question is that: can anyone really live with the former “love of his/her life” for 5 years, share special bonds for 10 years, and finally love another person? Can I really believe that she really loves me?

What hurts me the most isn’t the bunch of lies she told me about the roommate’s identity, but rather the single truth that they once shared something very special that I never could. Her ex/roommate had supported my fiance a lot financially and stayed with her through all ups and downs, and I’m not confident that I will ever have a chance to do the same in my lifetime.

I really love this woman, and I don’t know what to do.