Are you using "Soft body self collision" in the physics tab of your tail object?
I tried to emulate your work. Here is an example of how the simulation looks with and withouth self collision:
Without self collision
With self collision
To simply explain this, self collision adds balls to every vertex of your object. This allows the vertexes to not collide with themselves and keep a structure.
Here is the old documentation, here the new one.
Also, a really important option is stiffness. A high stiffness will make your vertexes more resistant to deformation, while a low stiffness will make your object deform more.
Here is the file with self collisions ON. Remember to change the settings of the balls on your file, as I did it in some seconds, and the balls would not be accurate with your setup. Just try and try again to change the size of the balls, until you get the result you want.