PE job sucks, how can I navigate returning to my old firm?

Hi all,

I was a Big 4 auditor for just over 2 years. I left for a job at a top-tier private equity firm for a significant pay hike, in an LP / portfolio reporting role. I've been there for three months and am hating it, the day to day of the role is extremely easy, and boring (I do maybe 15 - 20 hours of actual work a week, mainly reviewing valuations and performance)

I have mostly automated things, and it seems there is little room for growth or building any new skills. A lot of the time I am teaching myself modelling and other things through the plethora of examples available internally (basically the only upside along with the pay). 

I am wanting to go back to the Big 4, ideally in FDD but wonder how I should approach it, given I just left the firm. An FDD partner already knows me, and there is an opening. While I was at my old firm, the plan was for me to move internally (possibly), but I thought this PE opportunity would be a good learning experience and more set in stone - obviously big mistake. Now I want to reapply but wonder how to navigate the inevitable conversation about why I'm deciding to leave so soon. Is it best to be just be honest and say I thought being in PE firm would be a good learning experience, but learnt the hard way that what you're actually doing is a lot more important than where you are?

A lot of people have mentioned just enjoying the high pay and little work, but I am more wanting to actually pick up some skills.

Any advice is appreciated. I am writing this with the assumption that odds of an internal move in a PE firm are slim to none, also curious to hear about what this role actually leads to long term in the event I stick it out.