Your removable retainer will often feel tight if it is new or it has just been adjusted. This is quite normal and after a few days of wear the retainer will not feel as tight. If you have left the removable retainer out for some time, the teeth may have moved making the retainer tight when it is worn. Follow the advice below if you think the teeth have moved.
If the retainer is neither new nor recently adjusted, it may have become tight due to one of two reasons. Either it has distorted in which case you should contact the practice for advice, or your teeth are trying to move to their pre-treatment position. If you think the teeth are starting to move, you should wear the retainer for longer, perhaps 22 hours in a 24 hour day. Contact the practice for advice.
Do not remove the retainer in the hope that it will be easier for you. The relief will only be temporary as you will have to start wearing the retainer again and the tightness may be worse because teeth may have moved.
If you feel your retainer is too tight to wear, contact the practice for an earlier appointment.