I do understand, but do you need a car for commuting that has the costs of the BMW associated with it?
Like you I spend along time in an ordinary car each day. My definition of ordinary from 2007 to 2016 was a Chevrolet Lacetti converted to LPG. I stopped using the main dealer after the last free service and started servicing it myself and the LPG conversion almost halved my fuel bill. I eventually traded the car when it hit 185000 miles and beyond tyres and brakes it hadn't cost me a penny. Ok, it's not flash or fast but it also wasn't expensive to buy or maintain and didn't owe me a penny even long before the trade in day. I replaced it with 2012 Kia Cee'd. Also an inexpensive car with cheap running costs that just does the daily grind and work horse duties expected of it.
I have taken this approach for years, a basic car for daily duties and put the money into something fun for the weekends. It's a compromise of course, but for me it means I can afford my 'nice' car and it's kept nice for when I can drive it. The other car, well who cares? It's just A to B at the end of the day, and it's makes the weekends that much more special.
It's no different to those trades who need a van, plumbers/builders etc, it's a work tool.
See my thread titled "hybrid comparison". Do I want to drive a hybrid or electric car? Of course not, but the money side side works and if the Kia was a year younger I would have kept it and opted out the company car.
BMW's are expensive to buy, expensive to run and have high maintenance costs. I know they also have high(er) residual value so sell it and use that to get something mundane that makes the Mustang even more special.