Pushover / People Pleaser

I am so tired of pleasing people and saying things to please them, even if I disagree. It's like others can sense this and say unkind things to me or boss me around. I can't even get the courage to til my MIL I don't want to go to their church in fear she will talk crap about me to her congregation or become very hostile. PLEASE - are there any apps, books, etc to help with this? I am tired of living this way and always anxious that I'll make people upset or hate me if I say NO.