A lot of overlapping faces

Hi, I try to 3D print this object.
I created it with curves joined them, turned them into a mesh and now all the faces are overlapping.
I tried to flatten the top and bottom with a boolean difference, but it doesnt work.
Already tried merging vertices but it doesnt help.
Sitting sinze 2 days at this problem and cant find a solution…

Heres the file: https://easyupload.io/r3g652

Using voxel remesh is the only way I know how to fix that problem. This of course would change your topology. You can try adding a remesh modifier, and choosing voxel, so you can see what it would look like. You might have to change resolution.