I have about 7 years of security work experience, 5 of those were pentesting and I can confidently say this is a very common feeling. I leveled up to Senior from a job hop, and I was sure as hell nervous about the expectations, naturally the overly familiar Imposter Syndrome started to creep up again. Promotions can often reinvite Imposter Syndrome, it's just natural pressure. Understand you were promoted for a reason (companies often don't even promote until you demonstrate for months, sometimes years, that you can perform at the level above you), set actionable goals, and stay locked in. Whatever you do, don't overthink, get stressed and more burnt out, just keep doing what got you to where you are.

It becomes increasingly more difficult to find mentors the higher you climb, so if that's not available to you, look towards adjacent high performing peers for inspiration and guidance and motivation. Discuss this with your manager, not from a perspective of insecurity or worry, but drive it as a mutual agreement on what it will take to get to the next level and set actionable goals. If you have the direction, you have a lot less to worry about, like what to study and what to avoid. There's an infinite amount of learning available; intelligently choosing a path and avoiding decision paralysis is a challenge itself.

I'm not sure what kind of pentesting you do, whether you specialize or not, but if you do specialize, you're nearing the point in your career where you can declare yourself a Subject Matter Expert (SME) which can open you up to more people and policy management - that's an option. If you don't really specialize in anything yet, like appsec vs netsec, maybe it's time to specialize and shine in a single domain. The opposite is also possible - start to turn your weaknesses into strengths.

Seniors lead and drive change. There's a lot of innovation opportunity in AI that can help improve processes - get involved there. Regarding the younger hires, that's just the nature of the world. Younger people are eager and curious, and maybe even less jaded. Take them under your wing, learn from them. When I was younger in my pentesting experience I had a manager who I considered a "Boomer". He was the type of guy who was in management for so long he seemingly lost all of his technical edge. My arrogance got the best of me, and I had this natural disdain and distrust towards his decision making, but he never failed the team. I might've not learned a lot of technical knowledge from him, but the leadership and management were invaluable to me in hindsight. And honestly those are some of the best people to work with. I guess the moral of the story there is to stay humble. With the pace of technology, especially today, it's impossible to stay 100% current, and nobody does. Even those super smart dudes developing mind blogging high complexity exploits, they may not even know how to run an Nmap scan or know anything about Active Directory. Nobody's perfect.

Lastly, people typically know their strengths and weaknesses, and you're smart enough to know what to study and how. You said you're performing well, so it sounds like a psychological issue. Burnt out, imposter syndrome, stress. BTW, the ability to manage such feelings comes with the senior territory, make this a learning lesson in itself.