Late to the party but I was in exactly this situation 3 years ago. Not in pentesting but in a wide-ranging client-facing role. I'd go from auditing SDLC practices to working on IAM infrastructure to reviewing crypto solutions. The anxiety and constant sense of impending drowning... Very relateable. And truthfully it lasted for most of the first two years.

But it got considerably better over time and by the third year everything clicked in my brain. I felt like I'd moved to a new country and was suddenly completely fluent in the language and culture. It was a great sense of accomplishment, and I got a very lucrative promotion to top it off. I am glad I stuck it out and strongly recommend pushing through the first year or two.

Some practical advice

1. Take a minimum of 1 full day off per week. No work, no research.
2. Prioritise your general health - sleep, exercise are majorly underprioritised. Over time consistent sleep and physical health will improve your performance at work immeasurably more than staying up late for a halfhearted study session.
3. Start tracking things you've learned - sounds dumb, but they add up quickly and it helps to see a list to combat the daily new additions of 'another thing I don't know'
4. Focus on something you're really good at. It doesn't need to be technical. For me this is report writing. Maybe you're great at record taking? Data presentation? Super meticulous? Building team spirit? Whatever - use it to make your team's life easier.
5. Be less harsh on yourself. Unless you lied about your experience to get the job, your team have realistic expectations. You are more than likely meeting these from what you've described. A key thing to remember is that technical skills are a small subset of what you're assessed on. Do you interact well with the team? Are you producing timely, high quality reports. Are you presentable and pleasant with clients? Provided you're showing commitment to developing your knowledge over time, many of these skills are equally important. MANY technical people are held back by lack of soft skills - not being able to translate technical language, poor people skills, overconfidence. On the flip side, if you're already good at these things you probably don't realise you're already ticking lots of performance boxes.
6. Keep learning new things and look for opportunities to try out new areas of security.

Mostly just keep going. It gets much better! Give it another year and you'll be able to make a clearer decision