You have answered your own question.
You want to stop being P/A, you have to pick. Passive or Aggressive. Ain't room for both in your skin. No one likes confrontation, yet without it, and respect to our own sense of self, we get walked over big time.
You are having issues with your boyfriend? Then deal with them. Tell him straight what is bugging you and why - and don't let him slip you some tired old excuses.
Stop being "nice" and start being you.
If you find it hard to articulate what the problem is when you're talking to someone, how about before you bring it up you sit down and write about it? If you have a few bullet points written down it might make it easier to get your point across.
Did I write this? lol.
But I know what you mean, I can be passive aggressive and I hate confrontation. But I've been starting to get over that, by speaking up. Really there is no other way to do it. I deal with it by having the discussion in my head first. I think of all the ways the person could react and all their possible responses. Also whenever I get scared to say something, think about how the other person doesn't know what's going on and that they've done something to upset you. If they really care about you, they would want to know that you are upset. Cause you have to realize that people don't always do things purposely to hurt you, they are just going about their lives and don't always realize how their comments/actions effect you.
So I say start small, an example is telling your boyfriend you don't like it when he puts his feet on the coffee table. Not a big deal, but the relief of saying it and knowing it's out there will be a big boost in your confidence. Which can hopefully lead to you expressing the more important frustrations you have.
Good Luck!!
i lost a friend because she hates confrontation. a really good friend. avoids it at all costs. then when issues come up she pushes them down and lets them build up and build up (for months and months apparently) along with resentment until some kind of passive aggressive comment comes out that ends up doing much more damage than had she just addressed the issue to begin with. nothing wrong with being real and owning who you are. i always say, your feelings are your feelings and even if people don't like 'em, their dislike isn't your problem. it's part of what being real is all about. i read a quote somewhere once, wish i had access to it now. it said something like, "people think avoiding confrontation is the key to a successful relationship. the truth is, dealing with confrontation is the key to a successful relationship. good relationships can't grow without it." all confrontation is, is confronting the issue. you CAN confront the issues without being aggressive about it. makes you, and the relationship in question (friendship, boyfriend, parents, etc) that much stronger.
If being aggressive bothers you, or you don't like being thought of as aggressive, be assertive then. Being assertive is just a way of being aggressive, without the jerkiness that's often associated with the word aggressive.
Confrontation is part of life, it's how you set and defend your boundries, without being confrontative, you're allowing people to walk all over you, take advantage of you, then you end up being that P/A no one respects. And when you do try to assert yourselve, no one is going to take you seriously, which in turn compels you to be aggressive. That's why its best to define and stand by your boundries in the beginning of any relationship, personal, love, professional, etc. "No" is not a four letter word, knowing when to say no is one of the key social skills for building respect for you by others.