How to stop being a doormat/ people pleaser?

Im naturally a nice person and i tend to avoid arguements. However, instead of people actually seeing me as a diamond in th rough full of nasty manipulative people they decide to use me. My old bestfriend tried to use me to hook up with my sista, a old friend started talkin 2 me when she broke up with her man, but as soon as she got bak together i dont hear from her in months. She also said last time we hanged out that shed call so we can go hangout (me, her , her sista and man) and she neva called. So the question is how can i stop being so nice and so dishonest to myself and be more ertive? cause sometimes i dont wanna say my opinions or views because they are different to the person im conversin with. yeh so any help? plz.