Many children and young adults have undiagnosed mental health disorders but in the case of schizophrenia, the symptoms generally don’t vanish in adulthood. Theoretically it’s possible, especially if individuals undergo intense therapy, but usually it does not happen that way.
The majority of individuals who ultimately develop schizophrenia first begin to experience symptoms in late adolescence and those continue into adulthood. Schizophrenia among children is rare. None of the symptoms you have described in your letter would lead me to believe that you had schizophrenia as a child. Having had a mental health disorder as a child cannot be ruled out but schizophrenia would not be, in my estimation, the most likely possibility.
Having been in therapy might have prevented the development of a severe mental illness.
You also inquired about having dissociative identify disorder (DID). The average age of onset for DID is between the ages of five and 10. A common symptom of DID is gaps in memory. Simply having memory loss, however, is not indicative of DID. Many individuals who never develop DID have difficulty recalling childhood memories.
Memory loss is also associated with trauma. Trauma is correlated with DID but one can have endured traumatic events without developing DID. Repressed trauma might be another explanation for your gaps in memory.
You may have had an undiagnosed mental health disorder as a child, such as DID, but it is difficult to know with certainty. One would have to conduct a thorough analysis of that time in your life, which might include interviewing your family members, former friends, former doctors, accessing historical medical or mental health records, etc.
You did not mention whether you currently have troublesome symptoms. If so, that might lend support to the idea that you had a mental health disorder as a child.
Consider consulting a mental health professional who could assist you in thoroughly investigating your mental health history. You may ultimately find that gaining definitive answers to your question may not be easy but it is worth the effort. I wish you the best of luck.