It's certainly unfortunate your Dad feels it is okay to talk one way to you & another way when he thinks you're not around. If for no other reason, the situation you describe is itself enough of a reason to not do so. I've always felt my father didn't think much of me either, although he never really came out & said so, at least not when I was able to hear (that I can recall.)

Parents all seem to assume they have the right create images of their children & that the children are obligated to strive to achieve these images. And when the children don't, then the parents feel justified in demeaning them or, in some cases, even disowning them. Parents give life to their children. But they have no right to define who those children will grow up to be. Neither do parents have the right to judge whether or not a particular child has succeeded.

I hope that you can find, within yourself, the strength to rise above what you overheard your father say, brokenentity. I know it hurts. But it really says more about him than it does about you.