I had braces when i was 10. i am now 24. i got a new retainer 3 years ago but have not worn it in a year. now when i wear it even for 30 minutes my teeth hurt. how often should i wear my retainer to get my teeth used to it again?

Do not wear that retainer. In the span of three years your teeth shifted even further, and now that retainer (an appliance meant to keep teeth in their place) no longer fits. Excessive forces by an ill fitting appliance can cause bone loss and periodontal issues. Please go back to you treating orthodontist to determine if this one can be adjusted or if you need a new one.

Start taking responsibility for yourself Retainers retain. They don't correct. By not following directions you have permitted destabilization. By forcing them in you are even farther destabilizing your teeth. See an Orthodontist to restraighten your teeth, then have new retainers made and wear them 24/7 for a year, then wear them EVERY night for only as long as you want them to stay straight.