i wish i had some great words of wisdom, posie, but i don't feel i do.
what i do know, is that setting and maintaining boundaries is a process, and it takes practice. it feels so foreign to us, so it takes time for it to feel normal.
i do believe it gets more and more comfortable, especially when those who would push against them finally accept and it's not such hard work anymore. that will certainly come for you.
also, being around people for which you don't have to do this because they just respect you all by themselves, is helpful. we simply need to surround ourselves with good and decent people.