This is one of many ways of LLMs are being crippled by terrible UI controls. You can't do simple things like edit the conversation history to make it forget things.
You can edit the conversation history though. You need to try alternative apps/UIs instead of the product websites like ChatGPT. Those are only for collecting more training data from users instead of being the most useful interface possible.
if you haven't already, I recommend trying the openai playground instead of chatgpt. It is the same underlying ai (i.e. gpt4), but you have much more control over the inputs.
Bonus 1: Since you pay per token, it's much cheaper than a chatgpt abo
Bonus 2: You can increase the context window dramatically (iirc 8000 being the max for playground, while 2000 is the max for chatgpt)
Facebook had a paper about "system 2" LLM attention, where they identified which parts of the input would be distracting for the LLM and just deleted them.