Just to be sure: Is it also impossible for you to lift one knee out if you move the other knee to the side under the cockpit coaming?
If this is possible, it will be the easiest method. One knee up. Same leg over the side of the kayak so you dip your foot in the water. Then repeat with the other knee/leg. You end up sitting in the seat with a foot in the water on each side of the kayak. If you do it on the water with no support, you will probably need to do some sculling for support.
If you can’t do it one leg at the time, and you problem is that you can’t keep the balance while you enter the reardeck, you can put the paddle behind you for support and rest one hand on the paddle on each side of the kayak while you slide up on the reardeck. It is easiest if one paddle blade can rest on the sea bottom with the paddle extended to that side (requires very shallow water and). But it will also help a bit on open water if you remember to turn the paddle blades parallel to the water surface for support.
If you exit at a low jetty, place one hand on the jetty and the other hand on the cockpit coaming behind you, directly over the centre line of the kayak so you don’t capsize the kayak when you push down. Then pull yourself out of the cockpit and up on the jetty. Having the paddle behind you with one end on the jetty will also help here, but you risk breaking the paddle shaft with your weight if you are not careful.