I got my braces off in January (I'm 27), and I was supposed to wear my retainers all the time for the first 6 months, then at night after that for the rest of my life. I've made it to the "only at night" stage, and I do notice a little bit or soreness when I first put them back in because my teeth have started to move through the day; nothing I can see though. I'd say wear them at night, the braces are so expensive, and it would be an awful waste to go through the braces and treatment, then not wear the retainer at all and still have crooked teeth!
Fire14-I think you can have a new retainer made, although I'm sure you'd have to pay for it by now, probably been too long to be included in the cost of your treatment. My SIL looked into getting a new one, they told her they'd just take the molds again then make the retainer, same as last time.