You sound young, which makes me wonder whether you find calling yourself bisexual, like having a boyfriend, "easy and comfortable." Of course it s possible that you actually are bisexual, but it seems possible as well that you are unwilling to look at the possibility that in finding yourself far more attracted to females than males you could be lesbian--something that apparently terrifies you so much that part of you would prefer to remain in a friend-zone status quo "relationship" rather than explore. Elton John used to insist he was bisexual, and was even briefly married to a woman, but the truth he finally admitted to himself was that he is and was all along homosexual.
Just something to think about. In the meantime, setting aside all issues of gender: Whether a boyfriend or a girlfriend, it s wrong, wrong, wrong to allow this person to believe you are romantically involved when you are not. Avoiding honesty is more hurtful than avoiding the hurt of a separation.
Best of luck.