That was very bad time for me, when i sold out my Bitcoin. When bitcoin price was 9k+ usd, then i sold out my all Holding Satoshi for my personal problem. But now current price of per Bitcoin is 41k+ usd.
At least, you can buy back bitcoin again when the price is down, so you will have a chance to sell it again when the price is up. I think many people already get the same experience as you, including me because I already sell some bitcoin at that price, but I still hold the other bitcoin for the next highest price. You can buy bitcoin at any price, and if you can hold it for a long time, you will be able to make a profit in the future. I think you do not have to regret that because I am sure that you will still have more chances to have bitcoin.