Why Declining An Open Relationship Was the Best Decision I Ever Made?

In 3-4 days, it will be 6 months since my ex and I broke up. To be honest, I already knew it would happen eventually and noticed all the subtle and non-subtle hints a few months before that. I kept telling myself I would be okay. She would blame her parents, her work and her grad-school online classes for being too busy. Her nagging parents didn't stop her from going out with me in the first place, so why having more reasons trying to spend a lot more time away with me when we would only see each other once a day in the weekend? I understand her predicament. As a best friend, I support her for everything. As a boyfriend, it tended to get lonely.