I do think you have had a hard time here. However your post has mixed up two distinct aspects of your bf relationship with his ex and children.
The part where he was decorating his children's home I wouldn't have any issue with and you have to just suck that up because it's for his dcs.
The part where his ex is constantly asking for lifts and seeing her everyday is definitely not the norm . He left (or she left) the marriage, he didn't leave the children. He needs to set some boundaries with her.
Does he have a set schedule with the children? Do they come to yours on a regular rota ? If so then that's what he should stick to at your home - where you can develop a family life of dad, kids and you. Not ex as well.
If he isn't prepared to do this then it's time to leave as it sounds like he still wants to be in the family unit with his ex and you will always be third fiddle .
Ignore all those who go on about 'like my own ' it's an expression that I understand given your circumstances and all that you do for them. You obviously aren't trying to replace mum .
In an ideal world you all get on, they PARENT together and you and he have a relationship together where you also help him parent when they are with you.
A year is plenty of time to be involved with the children ! Six months together is pretty standard for introducing, and if you are seeing them a lot , as your OP seems to say, then by a year you are a pretty regular fixture in their lives. So I do understand It will be hard to leave but unless he starts to separate from the physical space of his ex - you are in for a very difficult time which will become worse when you want your own children.
Who left who ?
Does ex want him back ?