I can sympathize with your situation. In my case, I'm the one that wanted to open up a 12+ year monogamous relationship with my still-monogamous primary partner. A few words of advice:
First, don't rush things. Monogamy is a huge assumption in our culture. It infuses almost everything and everyone we interact with. Processing that amount of push-back takes time. It was more than a year from the time I discovered polyamory until my partner felt comfortable with me seeing other women (we stayed monogamous during that time). You seem fairly motivated to move a bit faster, but values rarely change overnight. Perhaps a period of monogamy will help you work through your feelings. You should discuss a timeline with your SO so you both understand the expectations.
Second, it is possible to be monogamous and have a polyamorous partner (the situation in which my partner finds herself). It isn't easy, but it can be done. It does add stress to both parties (stronger jealousy and fear for the mono, guilt for the poly, and even stronger social disapproval), but it may be a solution that works for you.
Finally, you may simply not be the right match for your SO. There is nothing wrong with being wired for monogamy; in fact, it makes life much easier in our society. Your needs and lifestyles may be too different to continue a relationship. Sometimes, the best thing two people can do for each other is recognize that they can't be together.