The reason retainers hurt so bad when wearing them is that your teeth are having pressure put on them to keep them in the correct position.
After your teeth get set back in place and are retained for a few months then the pain should go away at least some.
If your retainer is hurting your teeth really bad or the pain does not get better after a few months then you should let your dentist know about it.
When you first get your retainer fitted you should wear the retainer for 22 hours per day everyday of the week and everyday of the month for at least 4 to 6 months.
Then after those 4 to 6 months of wearing the retainer 22 hours per day all the time you can switch to wearing the retainer for only overnight or only 8 hours per day.
By then not wearing a retainer for 2 days is usually okay and your teeth should not shift back out of place.
Although my dentist told me it would be okay to take the retainer out for 2 days soon after I had the retainer the dentist said that while I could take the retainer out for 2 days there would be a possibility of my teeth shifting back out of place.
So I opted to just leave my retainer in for at least 22 hours per day for 6 months.
If you stop wearing your retainer before your teeth have had time to set into place then your teeth can shift back out of place defeating the purpose of the retainer in the first place.
If your teeth shift back out of place after you stop wearing your retainer then your teeth will require new orthodontic treatment to fix your bite.
Then you'll have to start all over again and wear the retainer longer.
For at least the first 4 to 6 months of getting the retainer you need to wear the retainer for 22 hours per day.
Then after the 4 to 6 months of wearing the retainer for 22 hours per day you can switch to just wearing the retainer for 8 to 10 hours per day or just overnight.
Most people just wear the retainer overnight which is usually enough to keep their teeth aligned after the first 4 to 6 months.