Daydreaming is perfectly normal. In fact, it is often the locus of creativity and can be an important way to take time out from stress. But daydreaming to this extent is not normal or useful. It is interfering with your life in a very big way. The problem is therefore not the daydreaming. The problem is that you are not able to control it. It makes sense to me that you have been diagnosed with ADHD since you are so easily distracted.
From your description, I’d want to also look at Obsessive-Compulsive Disorder (OCD). You remind me of people who report that they can’t stop playing video games or can’t stop thinking about something they wish they had done differently. OCD is often characterized by the inability to stop a thought or behavior. It’s as if the “off” switch in the brain doesn’t work reliably. In your case, you can’t shut off the daydreams.
Dr. Eli Somer, a psychology professor from the University of Haifa in Israel coined the term”Maladaptive Daydreaming” (MD for short) in 2002 to name your kind of obsessive daydreaming. He and other researchers are trying to get it recognized as a psychiatric disorder in the Diagnostic and Statistical Manual (DSM). In one article I read, Dr. Somers says that people with MD frequently have Obsessive-Compulsive Disorder and/or Depression. You might find it helpful to search the internet for more information about MD.
As far as I can tell, there is as yet no certain treatment for MD. However, some people have been helped by treating the OCD and depression with a combination of medicine and talk therapy.
I wish you well.