It is too worrying or unpleasant for me to think about something that might happen or might have happened. I dread to think what my boss will say when I tell him I lost our biggest client.
I shudder/dread to ˈthink (how, what, etc....)
(informal, often humorous) I am afraid to think or ask myself about something, because the answer might be terrible or unpleasant: I shudder to think when he last had a bath.‘How much more work is there?’ ‘I dread to think!’