[Death by Mirrors]

From your description it sounds as if both threads are roughly (yet not perfectly) balanced, but both are too loose, and your bobbin thread is a bit tighter than your needle thread. But yeah, manuals usually come with pictures and that's the most helpful for figuring out your particular problem.

Sometimes however tension issues don't even arise from tension itself, so if absolutely nothing helps, you might want to check if your machine is threaded correctly, switch to a different needle, or - if you have one - a different bobbin case.

And yes of course handsewing is also always an option.