I'm not sure if this fits here, sorry if it doesn't.

I'm in an open-relationship, my partner and I talk about people we go out with and support each other. The problem is not with my partner, it's with someone I got involved with.

Ever since the beginning I made it clear that I was already in a relationship, she said she was ok with it. Later I discovered that it was because she just ended a very long relationship and was hoping to get back together with her ex, but things changed.

We fell in love, we talked everyday, saw each other every weekends. I was really happy with her, she seemed to be happy with me as well. I told her that I loved her, she told me she loved me back.

We had a pretty nasty fight, well, I mostly listened; she told me that I lied, that I used her, that all the moments we shared were nothing but lies. I had already told her that what I call love goes beyond just romantic love, it involves friendship, caring, wanting to grow up together, affection. But I know nothing lasts forever, that feelings and people are subject to change, my love wouldn't be the same forever. Does this mean that it's not love? If it hurts someone like that, can it really be called love? It was never my intention to do so, but I hurt her all the same.

Edit: I asked her if she wanted to be in an 'official' relationship with me as well, but she said she was fine with how things were.