You have not failed him. You loved him the best you knew how. He just does not know what to do with love that is given to him other than use it and abuse it. He does not understand it or see it as the gift that it is. He seems to think he is "entitled" to it to do with it what he will and has no idea or need to return it. That is a character flaw in him and has nothing to do with you or any other woman he may have a relationship with (other than MIL who somehow instilled this in him with the way she raised him). You need to get over this thought process and now as Kendall will need you to show her how to deal with this when he starts it with her.
Once again, you are being too hard on yourself. It is not your fault or problem to solve. It is his and until he chooses to become a person his daughter can be proud of he will not change.
Let him go and be happy with what you have. None of his actions or those of his parents have any reflection on who you are. Focus on the fact that you are a good person who is loving and kind and has a big heart BUT one who has learned that life is too short to waste all these great qualities on someone who cant see them and cannot appreciate them.
