You must go back to your dentist and tell him/her your new denture is not fitting well. S/he may be able to adjust it, or have it adjusted. Unfortunately it can be difficult to get lower jaw dentures to sit firmly. Ask if there is a fixative your dentist would recommend, perhaps?
Also, if this is a very new set, or if you have recently had teeth removed, it may take a few days for your mouth to adjust and any swelling to settle down. But pinching or pain you shouldn't just put up with. Ask for help.
Rita are living alone independently? If you have a relative or caregiver ask them to help you with this problem. Dental work is very expensive so you have a right to get it done properly
Also, if this is a very new set, or if you have recently had teeth removed, it may take a few days for your mouth to adjust and any swelling to settle down. But pinching or pain you shouldn't just put up with. Ask for help.