I met my current partner and realized he wasn't like all my previous relationships where the guy latched on in a monogamous deathlock, crawled up my butt and built a nest. He didn't require monogamy as long as I didn't lie about what I was up to with others and preferred a relationship model that allowed him the same. I wasn't his first attempt at vocalizing this to who he was seeing but that attempt had failed on him. Most of his relationships ended over cheating.
I was attempt number two and he wasn't good about disclosure so we had lots of rocky situations getting to where we are now. I did all the usual self dissection looking for inadequacies in myself that you're doing. I experienced real jealousy for the first time. I kept mental records about things I did for him and stacked them against this issue I believed he had in not needing me to be monogamous to him and wanting to not have to be monogamous to me. I saw it as this relationship's version of me putting him before me and him not doing the same because all of my previous relationships had that theme in one way or another.
You have to challenge those ingrained assumptions at least once in a while.
Such as this thought pattern. In essence, I was saying that because I was willing to be monogamous, which I don't struggle to be if it is expected out of me, and I was calling that something I was doing FOR HIM and resenting that he would prefer to not be monogamous with me.
But when your partner has no problems with you wanting and having sex with others how is being monogamous something you're doing to put them before yourself?
And when I took a good long look at that I also saw all the ways he did put me before himself. Whole lists of things he does that he would otherwise not do if not for me. Not right out of the box but more and more with time as he learned what ways I needed and wanted to be put first. He is amazing at it now. And while he would try to be monogamous if I demanded it, we both know he would struggle with it and eventually let me down in some fashion with his interpersonal relationships.
It comes down to finding out if someone being monogamous to you is the only way and/or a much needed way for you to feel that you are being valued the way you value your partner. There is nothing wrong with putting your partner first if they are someone who will do the same - IN THE WAYS YOU NEED to be put first. The second and much more enjoyable leg of this race is finding out what you can gain in not having to be monogamous either.
THEN you'll know if you're monogamous.