I don't have any old fics that I regret not finishing (ones I regret writing? Now that's a different story). Most of the things I write that I wish I would've finished have all been ones I've written within the past few years.
Sowing Seeds
I would love to finish the Shadow the Hedgehog fanfic I was working on, which gives Sonic Boom a well-needed makeover, and integrating Shadow's backstory with Maria into it. I love writing that one because Shadow has such an eloquent way of speaking/narrating and it involves a lot of inter- and intra-personal relationships and introspection.
Annie and Six
I regret never even getting close to finishing the original joint story of Annie and Six, but I think it needs a huge rehaul at this point. Similar beats, but a different setting. That undertaking needs to be done when I have a little more time to myself.
Forever Together
I regret never finishing my Forever Together story, about a megalomaniacal dimensional traveler who enjoys dimension-traveling to tearing down and rebuilding societies in his image until, inevitably, he is overthrown. But it's cool, cuz he's just do it again, np. He meets an outcasted, genius "technophile" in a world where technology is feared. She and he basically team up to create a superpowered society in their image, and he falls in love for the first time, with her. I love the tragic ending and the way the ending is actually chapter 1.
Entwined, Always
Another fanfic I regret never finishing is my Lucas Baker/Resident Evil 7 fanfic. I worked on it for 2 years and got really close to finishing, before I burnt out. Persephone, the second main character and Lucas's love interesting, turned into such a fun character to write. Lucas is a lot of fun to write for, too.
Cassie's Reverse-Harem
And I think the last one I regret not finishing is another original story. It's a sort of reverse-harem style story, wherein the main female character, Cassie, has several suitors: Saul, a common-level demon, Ezekiel, the literal demon king, Kyle, her long-lost best friend who moved away for over a decade before coming back for college, and Kane, her other best friend who she's known since she was a baby. This one is more of a fluff-filled story with some creepiness thrown in cuz, you know, demons. The main conflicts are just relationship ones, with one of them being a main conflict for Saul and Ezekiel involving one of the Demon Families. There is a lot of religious imagery in this one, as well as ghosts and the afterlife. Saul is such a charming guy, but I think Cassie needs a rehaul to make her more interesting. Her personality is mostly dictated by her studies and interests, so I need to do better with her.
Your story sounds so cute! I hope that one day you find the time to recreate it now that you're more experienced. That's one of the most fun things about writing, imo!