How can I word this?

[FuckingNoise]

I am in a senior role at work (clerical) and for a number of reasons I am very unhappy. The company itself is great and I'm treated with respect, but the role and the working conditions are no longer compatible with my personality and the older I get the more introverted I am becoming so this will only get worse. It's not that I can't do the job, I'm often praised. It's just making me unhappy and I can't go on faking it like this. So I'm looking for a quieter role with a smaller company. I'm willing to take a pay cut if it means a fresh start and a better environment for my mental health. I'm not massively ambitious, I just want to be happy. The thing is, how can I say all this to a potential new employer without looking under confident and weak? Should I be honest or just hide the fact I'll be stepping down and use a relocation excuse or something?

Anyone?

I see no reason to tell a new employer anything.
People move jobs for many a reason.
I haven't had a great deal of interviews but when I have been asked why I want to work for said company, I give a relevant answer ie. They are a leading IT outsourcing company blah blah.

Just as @NiceTwin says. Only apply for a job you think is suitable and if they ask why say you want a new challenge and whatever attracts you to the company. Less about you more about them. Sounds like you've been with your current employer for some time so that will ring true. I don't think they'll really care 'why' they just want to make sure you are a good fit for the role.

I've been there since 2009 so I'm a very stable employee