I know where you are right now, ive been there about 4 times in the last 9 years ive had my escort, but every time i drive the car iīm glad i dident sell it or break it.
But itīs your mind, but you will regret buying an m5 as you cant do to that what you have done to the sierra.
You have the car just as you want it now, and itīs gonne be HARD WORK to get another like that again if you regret the sale.
I normally leave it for some months, go and look at it, and then i think of the good times and normally that gets me going again.
I spent 4 years trying to get a management system to work properly, and almost gave up but in the end it worked out just as i wanted it.
So keep it, but put it away for some months is my solution