It's not naive to think that way so much as it is that you have idolized him. This is common with BPD and you will get over and past this. The thing is, if you can, think about how you think he is the best man who could ever be with you and then think about the primary criteria for the best man for you - that would be one that chooses to be with you first and exclusively. He has taken that away and has chosen to go his own way. So the primary thing that would make someone be the best one for you cannot any longer be that because he doesn't even choose you anymore. No matter what other criteria you may have in your head that he fits, "great potential father", "wonderful lover", etc.. it no longer fits because if they don't choose you, they can't be the best person for you anymore. he chose not to be.