Nothing really tbh. I mean, I could say not buying the Solid Golden Base Set Pikachu for 2.6k USD as pre-release back in 2016 (including middleman fees, shipping, import) since it’s worth 12.5k+ USD now; or I could say buying the Pokémon with YOU Wagon promo for 950 USD when they started popping up for the first time at the start of 2017 since its release in 2014, since it’s worth only 150-250 USD now. But no, I don’t really regret those decisions.
When the Solid Golden Base Set Pikachu was released at the end of 2016 my most expensive card I bought at that time was ‘just’ 300 USD. I had also just bought my first car for little over 4.75k USD. So I wasn’t willing to spend more than 2.5k USD on a single card, even if it was a Base Set Pikachu made of actual gold. Did I expect it’s current high price? No, not at all. Do I now regret not buying it? Kinda, sure. But: do I think I made the wrong decision back then? Not at all. Do I regret that decision I made back then? Not at all.
Similar with the Pokémon with YOU Wagon promo. It was released in 2014 (given to kids that survived the tsunami and earthquakes of 2011 in Japan), and none had popped up for sale for years. So when one popped up for sale at the start of 2017, everyone who was looking for it jumped on that auction. That first one ended for 1.35k USD (excluding fees), which I unfortunately lost. The next day, another one popped up for 950 USD buyout, sold within an hour. Third one that popped up I placed a bid on of 777 USD. Before that auction ended, ten more had popped up including some with lower buyout prices than my max bid. I ended up winning that third copy for 770 USD (7 USD apart from my max bit, rofl 
), which ended up being 950 USD including middleman, shipping, and mostly import fees on arrival. Within half a year their price dropped to 300 USD due to the flooded market, and nowadays they are worth 150-250 USD, and it has been around that price-point for the past 1.5 years. Did I expect it’s current low price? No, not at all. Do I now regret buying it? Kinda, sure. Would have saved a lot of money if I was a bit more patient. But: do I think I made the wrong decision back then? Perhaps. Do I regret that decision I made back then? Not at all. What if those three first auctions were the only ones that were popping up, and no more would pop up for years again like the three years prior. I would really regret not buying this amazing Pikachu card for my Pikachu collection.
We can’t change the past, so what’s the point of regretting something anyway? And although we can’t predict the future (this pandemic and the recent price increases are something I don’t think anyone could have predicted a few years back), we can still learn from our past for the future. To give an example: I’ve only bought a single Mario/Luigi Pikachu cosplay box and the XY-P Mitsuhiro Arita Pikachu as pre-order for my own collection for about 30 USD each. Nowadays they are worth 1k+ USD each. Do I regret not buying 100 copies so I could have sold them now for a huge profit? Sure. But the same can be said about not buying that winning lottery ticket. Or not buying hundred Base Set booster boxes 21 years ago. I now just know that if a similar promo is released, I just have to buy more than one.
Instead of regretting it, it’s better to learn from it. 
Greetz,
Quuador