I'm non-confrontational too for the most part, but not passive. While I *am* an introvert who's happiest with a comfy desk chair, some good music and a really great project to work on, when it comes to working with other people I found I had to be really flexible with my approach and also tailor how I react to whom I'm speaking with. We're both out now but just in case you ever feel like diving back in, here's what I found I needed to do in case it could be of help for you or anyone else reading this.
Most of the time, if things are going well or something comes up as a change that is (in my own professional view) not really necessary but also not harmful to a project- just go with the flow. However, if you see a problem it's your responsibility to bring attention to it. And that's where it can get really tricky.
As a societal construct, for the most part we are not taught to use the word "no" until it's at the point of becoming a personal defense mechanism. For many women it's a showstopper word, unladylike, hostile and just not part of the daily working vocabulary. It's also one of the most powerful words to have in your arsenal when it's needed, when you follow it up with "_why_ this won't work" *and* (most importantly) another option or an alternate way of looking at the problem if the person you are talking to doesn't quite understand what the problem is. Professionally, it's my least used word as people (men and women both) tend to remember the few times you say "no" a lot more then the majority of the times you say "yes" as it's unexpected, and it *will* shape other people's perception of you, but I just chalk it up to one of those necessary tools that's needed when working with people.
It's a magic word that helps stop project eroding things like scope creep, personal work overload (as you know better than anyone else how much you can take at a time, though with the nature of the industry be expected to use that rarely since high workloads are part of the territory) and keeping yourself on an even keel with the more extroverted co-workers. It doesn't come close to "fixing" everything, but there are situations where it's an absolute must.
Try saying "No" out loud. It sounds weird to say it. This is going to sound goofy, but practice how to say it "flat", with very little emotion behind it, and in conversation be ready to back it up with at the very least a "why". Don't use it unless you really have to, but be ready to use it. And when that still doesn't work and you don't have the attention right then, be ready to walk away. Not in a huff, no parting shot, just a simple "we don't have time for this now" or "let me know when you want to discuss this" will do -- even *over* what they are saying, and then do it. Chances are very good it can either be resolved in email, or at a later time when the other party is willing to listen.
I realize this sounds like a luxury or risky. It sort of is. Position matters too, as a trainee or junior programmer I would have never thought of using it (or when I had to convey issues- I used "soft" words), but as I got more responsibility I had to in turn become more assertive on scale, simply due to the nature of the job.
But sometimes no matter what you do, you will still end up with a full loss situation (as opposed to a compromise, which was what I generally aimed for with conflict- even while using the word "no" as a tool) and that's just going to happen. It happens to all of us.
There's a whole other flipside to this- and it's about learning how to sell your ideas, but this has already gone into journal length and well, we're on Slashdot which probably isn't the best venue. There are probably some great resources out there for that (though I don't have any on hand right now) but it's worth looking into if you're ever up for having another go in the IT world (or just have to pick up a job in addition to your business in order to make ends meet).
But to wrap this up- if you can find a way to use both your passive/introverted traits but also incorporate some assertive/extroverted tools, you'll probably end up with at least a more satisfying or at least manageable environment.
Best of luck whatever you decide to do.