I sold at a loss and want to buy the same stock at a lower price, how long do I wait for the cost basis to adjust where I re entered at?

I sold like two weeks ago because it just kept going down and I got tired of seeing it go down. I notice if you sell at a loss and buy back in a day or week later, the cost basis is way up and it shows I am down a lot. I don't want to see that red number next to the P/L%. Is there a certain amount of time one must let pass for the cost basis to show where you re entered? I am using Think or Swim from TD Ameritrade. Thanks.