I can definitely relate to your situation.
Expecting them to understand your situation will only be an uphill battle,as they probably don't realize the intricacies of maneuvering through consecutive days,turning into months and adding up to years of pain.
A person's body can become dependent on certain controlled meds,as this is exactly what they do;control the pain,which in turn controls your levels of stress.
I would speak with your prescribing physician about your tolerance level,as your parent's will be of little help in this particular instance.
You stated that your parents have your meds.I was wondering if there was any particular reason for this.Are any of them health care professionals?
I can see where their "tough love" skills could be improved but alas,it is what it is.
Maybe a book on chronic pain would help them see the point of view that you are painstakingly(no pun intended) trying to place in their sights.
By the way,Happy Belated Birthday!!!