I get that its hard. It just is. There are thousands of fellow single parents on here who have been doing it completely on our own for many years (me 12 years). We just have to get down and get on with it. Its not fair, its not easy, but that is what we are dealing with.
We all have to do all the school runs, buy all the uniforms, all the shoes and trainers with no help or support.
Now is the time to focus on you and the children and stop feeling as though you are owed something from the mum because by the sounds of it, that will never change.
You have done absolutely amazing to keep them in school and give them some stability. You are incredibly fortunate to have a room at your parents place temporarily.
Now you have to focus on the future, forget trying to get money out of someone who is never going to give it, and work out the long term solutions for you and your lovely children. You can do this.
Go no contact with your ex, it will be much better all round for you and the children.
Focus your time and energy on what you can do for the children. Be persistent with housing, and in no time at all, you’ll have a house and a job and you will all be settled.