Self Esteem

Why is my self esteem so low? It is a question I ask myself and I genuinely can't answer it.
To people looking in at me I seem self assurred, sarcastic, in control and confident. But within myself I am none of those things. I am here tonight feeling like a fraud and a phoney. I am just waiting for someone to tap me on the shoulder and say, we know you are pretending.
I had to run my first full day pd session on Friday. I was terrified. But it all came together and I had some really great comments. But when I look through them I focus on the negatives. Why?
Why do I feel the constant need to de-value myself? Why do I never feel good enough? I never feel personally attractive or worthwhile, and never believe the good things people say. I am always the first to put myself down.
I think I do it to protect me. If I am the first to say it, if I don't expect anything better, then I can't be dissapointed. No one will be able to hurt me if I do it first.
I know I have to stop. I know I have to feel worth it. I know I have to love myself before anyone else can love me. But how do I reverse a lifetime of self doubt? How do I start to believe?
A very good friend of mine says that I am so terrified with my life at the moment because I don't know the end. (I will always read the last chapter of a book first..never really thought about why..just needed to know so I didn't get too concerned or something)So here I sit not being able to read the end. I don't know what is going to happen, how it will pan out....who the hero saves in the end. I guess that again is the journey that I am on. I don't have to know the end. I don't have to have all the information. I just have to have aptience and faith that I am a good person and I will get by. I have to start believing that i am worth it and I have to stop the negative thoughts.
Gee, lucky me, another goal.........