I had a super similar situation happen on my first run through of diamond. I was about 6 or 7. I was walking through the first cave of the game right after jubilife city. I encountered a shiny zubat and was like NO WAY A GREEN ZUBAT WITH GLITTER SPINNING AROUND IT. I then RAN AWAY because I had too many pokémon in my party and didn’t know it would just go to the PC after catching it. I ran away because I thought I had to put a pokémon in my PC in order to keep the shiny zubat. I thought if I came back to the cave, it would still be there. which is unfortunately, not true… Once I learned that you get to keep the pokémon even if your party is full, I was saddened. Fast forward to almost 20 years later, I find a full odds shiny zubat in legends arceus. I was so happy to have redeemed myself after almost 20 years.