I am a monogamous girl and fell in love with a poly guy. I am a very insecure person and it hurts me just thinking about him being with another girl but on the other side, I understand this is who he is and it doesn’t make his love for me any different. I want nothing more than to be with him, but is it truly possible for a mono and poly person to be together? Any advice would be greatly appreciated!!
The world is a rich tapestry and almost anything is possible. Could *a* mono person and *a* poly person be happy together? Entirely possible. Could *you* be happy with *this* poly guy? Sounds a lot less likely.
It sounds like intellectually, you understand how his polyamory functions; but emotionally, it causes you a lot of pain. That's okay. There are a lot of instances where we 'know' something but still feel differently. It may be that you can't logic and reason yourself out of being monogamous and really needing a monogamous relationship to feel happy and safe. If being with someone "hurts you," then you probably shouldn't stay in this relationship. Incompatibility sucks, but it happens.
If you want to try and work on this, consider moving out of the framing of "I am a very insecure person" to "I struggle with insecurity." It's not an inherent part of who you are, it's a feeling you have that can be sat with, worked on, interrogated, etc. Ask yourself where those feelings come from, whether they reflect reality, whether they are serving you, and what alternatives might there be. (Even if you don't decide to work on this relationship, that sort of reframing will serve you well in all things.)
But honestly, it sounds like a core part of who you are and what you need in a relationship conflicts with a core part of who he is and what he needs in a relationship. Don't do yourself the disservice of trying to silence and ignore your very real feelings and needs with over-intellectualizing and excessive sacrifice. If it's hurting you, let go of it.