i've also been at my current job for 2.5 years and it's starting to get boring. it's like the honeymoon is over :) i was talking to my dad about it the other day and he said that now that i've shown that i can do the work it's time to start working on my people skills. which i think is a good idea and might also help you. i'm not very assertive, either, and my job is sort of behind the scenes, but it doesn't hurt to know that you can look out for yourself if you had to.
you're probably right about your outlook stemming from your family's arguing style. that makes a lot of sense. but i think that everyone who works in the corporate world worries at one point or another that their coworkers/managers are criticizing their work or talking about them behind their back. and i also believe that a lot of companies thrive on this insecurity. where i work gossip is a big issue because it's 'kill or be killed'. so is passing the buck - it's easier to blame someone else than to take responsibility. (both of these stem from insecurity.) that's why i think, now that your job is getting repetitive, it's time to focus on building your self-confidence. if your boss criticizes your work, you should be confident enough to recognize that it's just work and not an attack on your personal character. like i said in my first post, some bosses are just like that and it has nothing to do with you. (it's how they learned to survive in corporate.) or if someone is talking about you behind your back, you should realize that their insecurity is the problem, not you.
i found a good site with some tips on surviving corporate culture: http://www.ichannel.ca/corporatepolitics. take a look and see what you think. i would also add one more thing: don't take it personally!