Don't regret that, never live with the past because you can't fix it. Instead, focus on the future and always keep in mind that it's past mistakes that change us and make us better. Maybe it could be bad for you if you didn't learn from that mistake, keep this in mind too.
Now, let's talk about gains. Bitcoin went from nothing to everything, from a few dollars to one hundred thousand dollars, this is basically x100K boost. Bitcoin will never achieve such a gain the next years because its market cap is already very high, billions of dollars. I expect the price of Bitcoin to double, triple or go up to a million dollars max.
You are right we should never regret on what we missed but I don't know why it is regretful whenever I think about it hehe, I am glad that I did not miss the opportunity to accumulate some BTC in the last year so I don't regret now as I am in profit now. We all must keep it up even smaller amounts can make us big money in BTC, speaking of making us double, triple, and hitting $1 million max is the optimistic price set for BTC.
But in the upcoming halvings, I think we can expect more, as like now we was not expecting BTC to hit $100k (at least I was not) same like this later in upcoming halvings we won't be expecting it to hit $10 million haha, I know that's crazy if BTC hit $10 million dollars. That's just gonna take too long, I might not be here, so I think the optimistic target for my age is $800k, I want to see more though. hehe.