Setting boundaries is for YOU and YOU only. Of course the WAS doesnt like it because it lessens (or eliminates if you hold strong to your boundary) their ability to walk all over you, call all the shots and remain in control of the R. Its not uncommon for the WAS to lash you at you and say you are being negative, mean, unfair or some other BS.
The key to boundaries is enforcing them. They are for you and your stability, growth and forward movement. And they are not easy to set and stick to (trust me, I know). You can set your boundary in a calm, polite and firm fashion. Just be sure not to react in a negative way when the WAS expresses their displeasure with the boundary. Remain cool and calm. Eventually it will become second nature to you and its an excellent way to help you move forward for YOU and your children.
If having the remains of your H's things in your home make you feel like you are still in limbo then pack them up and tell him to rent a storage unit or move them elsewhere. I packed up all my H's things he left behind (I did that for ME though, not as a favor for him) and put them in our storage unit. It was kind of sad and empowering at the same time. I was tired of looking at them and I wanted OUR place to become MINE. As I told my H you are welcome to come home and live but if your choice is not to do that then MY home will not be your free storage unit until you are done drifting from place to place like some sort of frat boy nomad. In 1.5 years he has lived with his grandparents, friends and on and off w/his girlfriend. It must be a terrible feeling for a 34yo professional man to have no home, no roots and no stuff.
Honestly, I still do worry about my H from time to time and how he is living and how little he will be left with but that is because I am human and not a robot. But, choices have consequences. Nobody told him to leave and not come back, that was his choice so he could carry on with his GF, so, it is what it is.
You are going to be fine. Yeah, it will hurt and be sad and you will feel rage and anger like you never knew you could. But one day the dust will start to settle (but may get stirred up from time to time and that is ok too). Stay on the path you are on for you and your children - nobody said your H cant join you if he so chooses but right now he just isnt "path material".
Hugs! PS - I am sad today too - this is the weekend of an annual charity event for my H's family. For the past 10 yrs. I was a huge part of it and this is the 2nd yr in a row I havent been there. I need to practice my emotional thought stopping because I keep thinking my H's GF has "taken my spot". Doest help that I am as hormonal as hell today. So, we all have ups and downs. I am thinking a martini might do the trick