I used to be in that exact position, basically the same thing you've described, so maybe I can help a bit by telling you how I'm starting to get past it. I'm still not very emotive, but instead of laughter and smiles being the only emotions people really see from me I have gotten better about showing and voicing my displeasure with things. It makes general social interaction much less awkward, less feeling like a doormat that people just walk all over because they think I'm cool with whatever shitty behavior they throw at me. I could be reading too much into your post and projecting my own thoughts onto you, so my apologies if I'm misreading you about the feelings of being being emotionally stepped all over, and if I'm wrong about that then that's going to make the rest of this post kind of weird, but whatever.
The real key to it all is that last sentence you left dangling by itself. I used to avoid introspection too. It's pretty hard to face inward and take an honest look at what you've got going on under the hood, so to speak. Everyone's got less than pleasant bits to their personality, but that's exactly why it's not such a big deal. Hell, even if you've got some super awful things lurking about in you psyche, just ignoring it won't make it go away. As the saying goes, the first step to solving a problem is admitting that you have a problem. You can't improve yourself without knowing what your problems are.
Anyway, that little tangent aside, if you can't be honest with yourself about how you feel how can you really expect others to figure you out? If you're suppressing your own emotions then of course other people won't be able to tell what you're feeling. They think you're just content or happy because that's all you show them. People can be remarkably dense about picking up on subtle expressions and body language cues, so it's no surprise that they fail to pick up on things that just peek out from underneath the mask you're wearing.
You say it's your own problem, and that's true, but it's only half the truth. If other people are making you mad or sad or whatever then it's also their problem. The tricky bit is that they might not realize it actually is a problem if you don't make it known, because people can be callous assholes pretty easily. This is where it being your problem comes into it: it's your problem that you don't make it known that they've done something wrong. If you don't express your negative feelings about things going on then you get shunted into doormat mode, complete with people stepping on your face and assuming you're cool with it because you just lay there and take it.
If you'd like to make that change, then step one is self-discovery. Go find somewhere quiet to sit and think about whatever random shit comes to mind, and don't just let the surface thoughts flit around and call it a day. Hunt down their source, figure out where they come from and what makes them (and you) tick. If you're mad about something, try to deconstruct whatever made you mad and figure out what exactly set you off. Why do you hate that thing you hate? How do you really feel about that one friend who's kind of a dick when you dig past the surface? Whatever the hell comes to mind, try and figure yourself out. Use the same sort of questions on yourself that you would when trying to figure out why someone else feels a certain way about something, and don't lie to yourself about it or it'll be useless. It'll probably take some time and effort to get past your own mental walls and get down to some good introspection, but trust me, it's worth it.
Once you get to the point of being honest with yourself (or mostly honest, because we all tell ourselves comforting lies to protect our self image, and that's okay) I would be quite surprised if you don't naturally start expressing yourself more. I found that after a couple months of setting aside some time each day for introspection I was getting a lot of comments from people saying I'd changed, which I suppose I had. When you get past denying what you're actually feeling it tends to kind of leak out on its own, which is apparently how emotions are supposed to work and people like you and I are strange for suppressing huge swathes of them. After you get to the point of getting to a reasonable level of honesty with yourself, it all comes down to building self confidence (to accept the fact that you do indeed matter and are worth something and don't deserve to go through life as a punching bag) and altering your habits (because breaking old habits is hard and requires persistence, dedication, and repetition to actually work).
The whole thing is far from easy, to be sure, but that's pretty much the norm as far as trying to improve yourself goes. If it were easy there wouldn't be whole sections of book stores dedicated to self help books. :P
