Understanding Taxes after separating with spouse

Last year was rough, with a lot of major life changes. My spouse and I split (though we have not filed divorce papers yet), we both moved to separate states, and I left one of my two full time jobs. Typically during tax filing, I traditionally tend to owe 2-3k in taxes, which isn't great, but it's easily managed. Apparently I fill out my W4s incorrectly, which I don't know how so, as I never claim any credits, straight 0s down the form.

FF to filing this year: she's filed her taxes. To be fair, she did ask if I wanted to file jointly still, and I did not get back to her in a timely manner due to an over worked schedule. She had someone prepare it and stated they filed her as single. Maybe I'm not understanding tax laws correctly, but I don't believe they should have done that, since we are not legally divorced. That aside, when filling out my own taxes there is a massive change in taxes owed. I'm using FreeTaxUSA to file, as we have for the past few years now. If I leave it as filing jointly, it's at roughly what I mentally prepared for:

2,198 federal owed, 1,724 state owed (state is a little odd here, typically tend to be owed back by the state by maybe 100 or so).

If I swap it to filing separate however, it changes to quite a large sum: 11,527 federal owed, 2,198 state owed. It's a difference of 9,329 federal and 390 state.

I understand it's typically better to file jointly while married, and I would have had I been better at managing my time, however since she has already filed singly that is out of the window. What I'd like to understand is what the difference is so staggeringly massive. It seems quite excessive.

Annual income was ~150,000, no investment earnings etc. (only recently started getting to a better place with money management due to lifestyle changes from us separating), no home ownership or other deductibles that would make sense. I'm contemplating getting to a CPA specializing in taxes to make sure I'm filling everything out properly, but I think it's hard to mess it up on FreeTaxUSA. I tried using their pro support line, but they were not exactly helpful. I checked if there would be a deduction for contributing to an IRA before filing, but that was a no due to employer retirement plan or something of that sort.

Any help would be greatly appreciated before I just dive into this large bill.