Can't remove Faucet filter/cap !

Hi Friends ! I recently installed Brita water filter in my faucet in the Kitchen. I had to buy the faucet cap adapter and fix it in my faucet to install the brita water filter. Now after a month or so, I would like to remove the Brita Filter assembly (not just the filter) and I did remove the filter assembly but I'm not able to remove the adapter that I had put it. It's so tight and I'm not getting a good grip to unscrew it. I tried to so hard some of the threads in the adapter (it's rubber or plastic, I guess) is damaged and I can't put back my Filter assembly either.
Can you please guide me on this ? Please let me know if you need more information. Thanks.
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