Thanks for your input, to all of you. It really helps me to sort out my mind and my heart. Lately I've been feeling some changes inside myself towards the situation. Is it temporary or am I making some kind of progress, only the future can tell. By contributing here you're all part of the shift, which I'm grateful for.
I did some more reading about poly vs mono, and feel that somewhere I start to understand the difference in how to approach both concepts of relationships. I realized that most of my problems were/are related to the fact that I try to apply my mono views on relationships to the relationship I have with her. By shifting that way of thinking, it seems to work much better for me.
Recently she was telling me she had a date with that guy, and instead of trying to understand why she does that and what the need behind it is, I just accepted it, but not in a bad, losing kind of way. Somehow I was happy for her. And I didn't totally felt angry or bad about it. I would lie if I told I didn't have any jealousy inside me, but it was constrained and controlled.
I also feel that I'm more open to different relationships with other girls without feeling a guilt towards my current relationship with her. She told me earlier, that she wouldn't have problems with me being with other girls, and having other relationships. At first I couldn't really understand why she told me that, even when knowing she is poly. It felt like she didn't care about our relationship. I don't see it in that way anymore. I know and feel she cares about the relationship we have. That makes me confident.
Still I realize it's all very new to me and there's still some way to go, but I feel I'm on the right path and right now I enjoy it.