I do this too! I've never spoken to anyone about it in case they think I'm mad, selfish, pathetic, etc. I'm in my 50s and have done this for at least my adult life, can't remember if I did it as a child. I didn't know it was called maladaptive daydreaming.
I usually have one dominant 'alternative life' going on in my head most of the time, but there are generally a few others, sometimes short-term or one off stories. The dominant one goes on for months, sometimes years, and when it stops it's usually quite abrupt and it might take a few months before I find a new long term one, though I'll have the shorter ones in the meantime. It's usually semi-romantic, with a much better version of me - confident, attractive, outgoing with a great job and lots of friends and a caring, interesting partner, but it involves other stuff as well as the relationship - I can spend days doing scenes where I'm out with friends, at work, etc. I usually choose a man that has a job in an area that interests me and build it around that. My plots are usually fairly boring, everyday life stuff, but I'm always a very capable, popular, successful, nice, confident person in them! I suppose it's a way of making right all of the stuff I think I'm bad at or opportunities missed - in real life I'm introverted, anxious, underachieving, and many other things I'm unhappy about. I also think I have ADHD and probably autism. I wonder if I'd do it if I was really leading a happy, successful life. I love being in my other worlds, but it also makes me feel depressed and lonely because I feel like I've wasted my life just dreaming about a life that I don't actually have and don't share with anyone. I've wondered if it's an autism thing, because in reality I find social contact and physical contact very difficult, but it's hard being 'different', so I think the daydreaming is a way of making a 'normal' life that the real me couldnt' actually cope with if that makes sense.
I've tried writing down my alternative lives but there's not really enough drama to make them interesting - for example today I've been replaying a scene over and over where I'm cooking dinner with my imaginary partner. I've tried doing it the other way around where I use my imagination to play around with a short story idea and create characters and I quite enjoy doing that.
I'm really surprised and quite excited to find that there's a name for this and other people who do it.