dissociative brain worms

The treatment to achieve FF and FM are literally the same, the only difference is when you stop treatment. In order to achieve FF, you need to first achieve FM. You can't fuse all your alters together if you aren't already functioning fluidly with mostly dismantled amnesia barriers.

And even people going for functional multiplicity will almost always experience at least one fusion during their healing. Fusion is a natural result of integration and healing, especially when processing the trauma held by fragments or less frequently active parts. FM is much easier maintain with a smaller system size. In fact, a lot of the people who achieve FF were initially aiming for FM, and they realized how healing fusion was and how much more functional they became with less parts.

Fear of fusion happens as a result of dissociation, it happens because your brain has wired itself to compartmentalize trauma instead of integrating it, and is convinced that this is the only way to survive. It also happens as the result of the fear of normalcy that many complex trauma survivors have.