The simplest solution for the short term is to put a ball valve on your backwash connection and shut it when you are filtering, opening it to backwash or pump to waste. This may be all you need if all the other functions are running OK. Ball valves and the necessary fittings are available at pool stores and plumbing warehouses, and at Home Depot and Lowes. Easy short-term fix and you can do it today to prevent problems with your water.

You probably have a damaged "spider gasket" in your multivalve, but it could also be the walls of the valve are damaged, or the springs are weak and damaged. Or, of course, that it's not assembled correctly. Sooner or later you should get this fixed.
Carl