Nobody knows it but me!!!!

Why is it so easy to talk to our selves to make up stories in our minds about how certain scenarios will go how conversations or meetings will be played out but when it comes time for those meetings or conversations we freeze up, our mind seems to be doing the two step out beside us and those conversations we have had in practice mean nothing. I feel this is normalcy for me I can play conversations I want to have with someone out in my mind for hours but when it comes time for that conversation I sull up I become the quite me the one that gets run over by everyone in site and nobody looks back to see the damage. I've been told countless times throughout the past few years almost 7 to be exact, that I need to start standing up for myself stop taking crap from everybody and there brother. But, as much as the conversations play around in my mind and I keep telling myself I'm gonna tell so and so what I think the next time they make me mad, or start talking to me like I'm crap..but nope still haven't said anything to anyone..so there stands the famous questions of when, why, how. When will I stand up for myself when will I finally say enough is enough and start making the played out conversations in my head into a reality, why haven't I done it yet?? I'm old enough to know better and still to young to care, if only this were true..growing up I come from a family that taught obedience and respect respect for ourselves, for others and especially those that are older than ourselves. So there comes one of my main problems I have respect for everyone and that makes it almost impossible for me to speak my mind to that person. But yet coming from a family that shows so much respect to everyone and is brought up to show respect how am I the only one that actually does it??? How am I the only one that can't wont doesn't know how to stand up to those who hurt me??? I think this comes from being put down so many times growing up, being belittled and made to feel worthless and so low that you feel lower than than dirt (totally different blog :)) When you get to the point of feeling this low you eventually get to a point of being afraid to speak your mind. I truly believe this is my problem.