Your namechange is most apt, but in all conscience I can't give a 'gentle' response to what you've posted as the profound damage that's been done to your dc by your relationship with their df and your 7 year relationship with this man is all too apparent.
Has it occurred to you that the money you've squandered spent supporting this thoroughly nasty, arrogant, and despicable twunt of a cocklodger could have paid for professional advice and support during those "divorce negotiations" and the "big work problem" that subsequently arose and that, more importantly, it could have paid for the considerable therapeutic input your dc require to overcome their df's abuse and aggression towards them and address the additional psychological damage this twunt's done to them?
Many high-achieving and highly paid women appear to be emotionally retarded when it comes to choosing partners. I know several highly intelligent women who have put their dysfunctional relationships with the opposite sex before the needs of their dc, with the predictable result that those now adult dc have nothing but contempt for their dms and, sadly, their own 'romantic' relationships are flawed, to say the least.
You know what needs to be done but, in case you've overlooked anything, here's a list:
1 Before you go home this evening cancel any direct debits or similar transfers you may have set up to fund his business and pull the plug on any credit card/bank account/mobile phone contract etc you've made yourself liable for.
2 When you get home tell him to pack a bag and leave. If he claims he has nowhere to go, that's his problem; it's summer and he won't freeze to death if he has to sleep on the street.
If he refuses to comply with your polite request, call the police and have him removed as he no legal or other right whatseover to remain in your home if you don't want him there and he can make arrangements to collect the bulk of his belongings at a later date to suit you.
3 Get his keys to your home off him before he leaves - if this is not possible, call a 24 hour locksmith and get the external/entrance locks changed.
4 As soon as he's out of your your property, unlock all locked internal doors and hide the keys.
5 Apologise to your dc for being Fuckingstupid taken in by him and allowing him to dictate terms to the 3 of you and assure them that you will not allow another man to move in to your bedroom until such time as they are grown and ready to set up home for themselves and that you will only countenance living with another man if they wholeheartedly approve of your choice.
6 Pour yourself a 
, congratulate yourself on a job well done, and update here.
Tomorrow you can begin lovebombing your eldest, strengthening your currently strained bonds with both of your dc, and seeking professional help to redress the harm that you've caused by failing to put their needs before your own.
In addition, and if necessary, work out ways to consoliate your debts so that the 3 of you can have a much needed holiday together in a resort/camp site where there is no shortage of fun activities for teenagers to participate in and where you'll be able to socialise with other adults/read a book/browse the shops/go paragliding or whatever appeals to you while they're enjoying themselves with their peers.
Your dc have spent their entire lives being abused and intimidated by 2 men who were/are singularly unfit to be role models, and have been forced to witness their dm being royally screwed over seeking to appease and placate their latest abuser.
It cannot be overemphasised that you owe it to your dc to ACT NOW to stop the rot and to devote yourself to their wellbeing for the foreseaable future.
.