There are two aspects of mono and poly that interplay in relationships.
- How many people am I capable of loving at a time (one or multiple)?
- How many people am I comfortable sharing my partner with (none or multiple?)
The first relates to whether or not a person is mono or poly. The second relates to whether the person prefers to date people who are mono or poly.
In your case, you're mono and you prefer to date mono. Your boyfriend is poly. That's an incompatibility.
In the end he decided he just wants to be with me. Though I am still not certain if I am happy with his decision. :I I can see the hurt in his eyes that he still wishes he could have her in his life as more than just a friend, and this kills me inside. I do not want him restricting himself to keep me around because he loves me so much and doesn't want to lose me.
Reading between the lines, it sounds like you gave him an ultimatum and he chose you. I'm getting that from the "doesn't want to lose me" bit.
Your boyfriend can't change who he is inside. His desire to date other people will never go away, even if right now he's decided not to date that one particular girl. By allowing this arrangement to continue, you're merely delaying the inevitable. This issue will come up again, be it in a month, a year, 5, 10, 20 years. Personally, I'm not a fan of burying problems under the rug and pretending everything is hunky-dorey. That's a recipe for capital-D Disaster.