You played offline gamble and you were lucky enough to have won 20,000 BDT but since you where forced to continue gambling, you lost 15,000 BDT from the money you won.. I guess losing 15k from 20k you still have about 5k BDT remaining. So you are still in profits.
Actually almost everyone that have been gambling for a long time have been through this kind of situation countless times. Some weren't lucky enough that they lost all the money they won including their own initial deposit.
OP compain is that he wanted to stop the game but couldn't because he has no other option than to continue playing till one of the gamblers will empty his bankroll. Assuming OP, stopped gambling when he wanted, he would have enjoyed his big profits. This is how gambling works, when you are in profit and don't stop playing, the chances of you running at loss will be higher.
OP was forced to keep playing, this is one of the bad characteristics of street gambling, those whom you are playing against, if they are more than you, they will never allow you to walk away with their money, if at the beginning, you won money from them, they will make sure you keep playing until you have lost that money including your own capital back to them before they can allow you leave.
If at any point you try to force yourself to leave when you still have their money, they may likely beat you up and collect all your money including other valuables like phones before they push you out to go.
When on the start, be extremely careful with the type of people you choose to gamble with, make sure it's people you know very well and they know you too, like your friends or something.
A total disadvantage playing on physical environment especially if they are shy type person which cannot decline to what other people want to happen. There's instances that we might lose back the winnings we got especially if we feel shy to stop.
To bad if that situation happened on where they came to the point they beating up the winner because their request didn't granted. That's why its good to play on online casino since we won't experience this unfortunate situations.
This is why I'm so careful dealing with before people before and don't play random guys around.