This is a difficult situation. There will be no easy way to face the impending loss. Loss is an unfortunate part of life.
There are healthy and unhealthy ways to grieve. Your inclination is to avoid facing the problem. That reaction is understandable and common but it has the potential to be detrimental to your psychological health. It is important for you develop the appropriate support system that can help you heal and grieve. Utilize the support of your close friends in AA. Ask for their assistance while you transition to a new therapist.
You might want to consider attending an Alzheimer’s grief support group. It might help to be in the presence of others who are facing a similar situation. You may want to consider these resources to learn more about Alzheimer’s.
You’re fortunate to have met a mental health professional who has helped you. Try to treasure the time you have left with him. Also remember that nothing and no one can take away the relationship that you have. Death is a separation from the one you love, not an end or erasure of your relationship. All that he has taught you will live on with you.