This is a common problem with company cars. It takes months between getting a new car and you starting to pay the correct amount of tax, because the change is not reported until well after the end of the tax year.
The value is calculated by the list price, multiplied by a factor dependent on the emissions - the P11D should show the workings of this. This is the same for the whole time you have the car, there is no deduction for depreciation. If you pay towards the car for private use, that is deducted and conversely, if your employer pays for your private fuel, you are taxed extra for this.
So for example, if you got your new car in November 2020, you say the cash equivalent was double, because the emissions were higher?, so you would have owed more tax from the date you got your new car, but HMRC won't have known about it until your employer sent in the P11D, which won't have been until April 2021 at least, and could even have been as late as the end of June this year.
So you had underpaid tax for several months after November 2020, hence your tax bill for arrears and the fact that you are now paying more each month going forwards due to the ongoing higher cost of your company car. Another consideration is that if you earn around £50k, you could also get tipped into higher rate tax, making it even more expensive as you are taxed at your marginal rate. But you can pay it by adjusting your tax code, so at least it will be spread out over the rest of the tax year.
I don't have a company car any more, but what I did find was that, when I sent my last car back, I was able to log into my tax account and tell HMRC straight away that I no longer had the car, so I wasn't being overtaxed, as it would have been nearly a year before my employer told them this via my P11D. So in future, you can self report, so arrears or overpayments don't build up like this.