The most important lesson I have learned (quite recently) is ignore the fiat price of BTC. Accumulation of BTC is what matters and I understand that was your intention but were unlucky to get caught out. What I do these days is split my BTC in to HODL and trade the later being my accumulation account. I used to go all in but the system taught me a few lessons which includes patience.
All is not lost though, BTC could fall back to your sell price and even way below. Remember when it first reached 20k and it slowly dropped all the way down to 4.5. A lot of people got caught out in that downward spiral myself included. That's when I learned my lesson. If there is no substantial dip then you will have to buy back in again. Another strategy is look for a decent alt coin to buy (maybe something newish) and before you know it you're back to where you were. You do have options.