In my case, it's actually from my childhood - I was probably around 13 years old. I had been playing Drakar & Demoner (the newest edition of which is called Dragonbane in English) as my first RPG experience, and I loved it.
I don't remember the exact details, but I remember drawing my own boardgame-like board with a landscape I could traverse with various locations (I remember there being a settlement, a lake, a forest, and a castle in a remote, desolate area). Then I made my own RPG-like rules for walking around in this little world. I don't remember any of them, but they were most likely very simplistic and procedural. But I know there were some kind of random encounters, and you could get gold for defeating them. And I also remember I had a few different character classes (this was in the 80's, so ninja was obviously one of them).
I don't think I intended the game as a strictly solo game, but you could play it solo, and I never played with anyone else. I don't think there was a specific goal per se, but I just wandered around, fought monsters, got more gold, went to the settlement to get healed, etc.
And I just loved it. It was so... I guess I would say "cozy"? I had my own little world which was exactly as I wanted it to be, where I could do whatever I wanted.
This was 35+ years ago, and I've never been able to recreate that feeling again even though I enjoy my SoloRPG sessions now too, of course. But I guess the magic of childhood was an experience enhancer which can't be replicated in adulthood...
What are your favorite SoloRPG memories?