Fear is a reoccurring companion on the spiritual path. It doesn't mean that something has gone wrong. The particular fear that you describe seems fairly common, but fears can come in all sorts of shapes and sizes; all sorts of feelings and thoughts.
How can I overcome this fear?
The overcoming of fear happens when we become curious about it rather than adhering to its demands which generally push the practitioner towards form-based experiences or sensuality - that is the habitual response. This only serves to cover over the those fears and the practitioner loses sight of some critical knowledge. However, approaching fear with curiosity suggests that there is another way to understand those fears. Therefore, fear holds previously unseen knowledge... knowledge about uncertainty and not just the uncertainty of our mental faculties, but the uncertainty of every single second that passes by.
When one consciously choses to look towards those fears, slowly they lose their power over you. One does not need to analyse why they are there rather, just to notice that they are present. You see, fear is like a snotty-nosed child trying to play a sinister melody with a trumpet, but if we don't turn towards its comical absurdity, we perceive fear as something much greater, something that has much more substance; fear then continues as a deep intrapsychic governor giving determinations on how we live our lives.
Fear only perpetuates when there is an applied force. We can recognize those applied forces in our thoughts and if we buy into those thoughts fear quickly takes the upper hand. One common fear is the fear of going crazy.
Is this a fetter? In particular, is it caused or conditioned by belief in self? Are dispositions part of self?
It's not quite a fetter, but a more pre-emptive response to the practice as a whole. The fetter model has its quagmires; it's not a solid base for practice, so I wouldn't hold that model too tightly.
The practice uncovers these deep fears and allows us the opportunity to view them in more healthy ways. It's time to start doing that now. This calls upon a certain degree of wisdom, perhaps some faith, and an ability to find some contextual awareness.
In an nutshell, the fear is the anticipation of its own ending, but it can only express this through negative thoughts. It is fearful because what lies beyond our internal conceptual world cannot be conceived by it, by the feelings it generates or by the thoughts it weaves.
The amazing thing is, we fear the loss of control but none of us ever had any control and never will.
How can I accept losing control of my mind and will?
Chiefly, you'll have to feel your own way with that. When you've accepted it, then you've cracked the entire thing!