This person's individuality will scare you at a deep level. Somehow, their sense of freedom, of becoming the person they are and want to be, will feel like a threat to your own sense of security in the relationship, and until you can consciously recognize this dynamic, you'll do what it takes to try to limit and control them -- including limiting their personal freedom and controlling the development of their personality, neither of which should be within your domain as their partner. But try you will anyway, through rigid adherence to whatever 'rules' you set up for the relationship or through creating a sense of commitment between you that's more like obligation.
But where's the fun in that? All you'll be doing is recreating an experience from your own, deep, long-forgotten past -- another lifetime, in which this person's personality or personal growth led them painfully far away from you. That left a vulnerable spot inside you that you decided to 'heal' by never, ever allowing that to happen to you again. But building up defenses around your vulnerable spots isn't truly healing them, and trying to keep your partner by your side through guilt, obligation or other heavy limitations will only work for so long. It would be far better to strive for a sense of intimacy and togetherness that's based on the mutual desire to be together and to create something that will satisfy both of you long-term. This will be a difficult transformation for you, as it will require that you face your fears of being left behind, vulnerable and alone, once again; but once you do face those fears, you'll be able to give your partner the freedom they need to be who they are.