Yup.But I don't think that my reasons for seeming nice are the same on the outside as they are on the inside. My niceness stems from fear of conflict, but on the inside, I'm enraged that I can't be more assertive.
The basic solution is, one day, to position yourself so you are not entirely dependent on authority figures. At that point, you won't see conflict as a situation that could conceivably, totally destroy you; you will have the personal resources to continue, regardless of their response to you. i.e., you can risk conflict as part of a negotiation to make sure your own needs are met.
This will help diminish the self-hatred, since you will feel stronger and more confident and capable.
And then you can be nice to people just because you care about them, rather than primarily as a tool just to dodge conflict.