A 52-year-old woman reports experiencing sudden, brief episodes of intense fear, palpitations, chest tightness, trembling, and a feeling of impending doom. These attacks occur unexpectedly, last about 10-15 minutes, and she worries constantly about having another one. She avoids places where previous attacks occurred. There are no specific triggers identified. Physical examination and baseline investigations (ECG, FBC, TFTs) are normal. What is the most likely diagnosis?