I used to be afraid of this happening after I saw a movie (Sci Fi not real) where the person had an alter that took over forever. So I asked my counselor about this an she said no it would never happen to me. that I only remain dissociated however long I am scared and such then when she has me calmed down I immediately am back to being in the here and now instead of past memories acting out. She said I could stop worrying about that. Alters never take over indefinately. They are there only however long the person is upset.