Hi. I see this is your first post. I salute you for being able to talk so openly when you can't know us so well (unless you've been reading for a while).
IMO, there's no need for confusion. Commitment to one person is apparently not important to him, and unless there is some kind of epiphany in life that brings a motivation to change (and to work at the change), it is unlikely that he will ever be ready for an open relationship with you. And if he did, could you trust him to remain faithful?
And then, there's you. You can't be feeling very positive about your role in this, nor liking yourself very much.
Having a best friend is wonderful. So is having a mate. But there are relationships which tear one down - in which one "uses" the other. A good relationship involves giving and receiving. A bad one just takes and takes, and drains your soul of vitality and authenticity.
IMO, you need to stop enabling him in being inauthentic and dishonest. He is not being a good friend (let alone a best friend), nor are you to him.
IMO, the best thing that both of you can do right now is to break off "romantic" relationships for a couple of years while each of you works on growth as a person. Then when a person seems to be someone you want to commit to, and receive commitment from, the relationship can be equal and not codependent.
Sorry if this is not what you wanted to hear.
Marge is the love of my life.