That is not how the pDex, or any dex without orderbook, works. The more impact your order has on the available pool, the lower the rate gets.
When you make a trade, calculate how many coins you more or less expect in return. If the suggested amount is way different from that number, don’t trade.
When things don’t work out that well, there is a chance, unless someone else is monitoring all trading pairs closely, you can buy back, the way @NickNack did for you this time.
Most important always check the numbers of a trade.