Any ideas as to why my cake continues to sink in middle? I can't figure it out. So frustrated.

Thanks in advance

The only thing I can come up with is that your cake is undercooked. Try to leave it the oven for a couple of minutes more and I think yoy should not have to problem anymore.

Try putting a flower nail in the center. It heats up because it's metal and it helps cook the cake in the middle.

If you open the oven a lot before it is done, it can cause it to sink.

When a cake sinks it's indicative of a slightly underbaked cake. Instead of using a toothpick to check for doneness, use a thin blade knife. It's easier to see moist batter on a blade rather than on a toothpick. Another way to test - press lightly on the cake and if it feels firm then the cake should be done. If your fingers leave an imprint, bake for another 5-10 minutes. Hope this helps!

Is your cake sinking before or after you remove it from the oven? It could be underbaked, but if it's sinking before you remove it from the oven, it is more likely over-leavened.

Does this happen with all different sizes and recipes of cake you bake?
Maybe your oven needs to be calabrated.
Maybe you are not putting enough batter in the pan
Maybe you are not baking it long enough.
With the limited information you have given it is hard to tell exactly what is wrong.