Career mistake - should I stay or should I go?

Hi, first time posting here...I really need some advice please. I've made the decision to leave but my question is this: should I stay in a job where I can actually feel 'skill fade' daily and it's affecting my mental health? Or should I resign and continue my job search with a gap in my CV? (note this isn't about money) For context: I've been 4 months with a new company as a HR Business Partner role. I was with my previous company for more than 10 Years, and held a BP role for the last 3 years of that. The reason I left was because I wanted a new challenge, not because I was unhappy. I believed my current role was part of the evolution of HR for this company from transactional to enabling. It has quickly become clear that while that's what has been said, it's not what they actually want. I'm a very well paid Advisor and Manager, but will not be given the resources (time, training) to achieve better results. I'm constantly firefighting. This is not the job I applied for, and to be honest I think they've been dishonest in how they represented the role. I have discussed my concerns with my manager and a trusted senior leader, both of whom accept the above. They also acknowledge they don't thing things will change and can't help me to influence the role HR has to become truly enabling. advice appreciated, thankyou!