Accepting unwanted change
Hi. So I started dating this boy in January, when he came to visit me and then ended up moving here. We had hooked up last summer and kept in touch both romantically and as friends, we both had other partners. When he came to visit in January, I was broken up with my two other partners, we was beginning to break up with his partner. Dating was sweet and great and then at some point we realized we needed to sit down and have the relationship status & poly talk, and that we wanted to create some agreements so we could start to build trust (to heal from an incident that had happened the previous summer). In that talk, we both said we just wanted to focus on building the foundation of our relationship, and that would best be served by not hooking up with other people for a little while, as we developed trust and communication between us. He specifically suggested we go 1-on-1, I agreed, even though I didn't need that from him and was open to hearing about things. At the time, he had a crush on someone who returned the feelings, but said he had no interest in moving that to a romantic place. he promised to tell me if his feelings changed about her.
Well, as you can guess, 10 days later he spent the night and made out with her. I was really hurt and didn't understand why this person would suggest monogamy and deny his feelings and then break our agreements without talking to me. What followed was a long and painful process of mistrust, renegotiated agreements that were then also pushed or outright broken. In then end I came to a place realizing that this person cannot keep agreements or commitments to be considerate of my feelings, and that I should not be in a serious relationship with him. He acknowledged the same and we agree to date more casually, and he is dating that woman with whom he broke my trust. She is in my friend circle. I had told him from the very beginning that dating someone else in my small town was beyond my boundaries. He has continued anyways.
Meanwhile, though all this, I realize I allowed myself to fall in love with him, and am having a hard time coming to terms with that he doesn't want to be serious with me. I want to be serious with him, though I don't know why, because his behavior has been hurtful and untrustworthy.
How can I adjust to a situation that from the get-go is beyond my boundaries and was founded on broken agreements?
How can I try to date someone casually when there is a difference in feelings and/or commitment level?
I really don't want to just break up, I would like to still enjoy the love, affection, and sex he is offering me. right now, i mainly feel sad, especially when he's with his other lover, mourning that he doesn't want to be more serious and committed with me.
Advice? Also note that he his moving back to finish school at the end of this month, so this is also about transitioning to being long distance.