Okay, here goes

[FindingPatience]

When am I being a good DBer and when am I being naive?

I've been DBing since August 2013.
Since H moved out, he became much more respectful towards me and more reliable for our kids. Things have been relatively stable for a year.
He has only mentioned moving forward on the legal process once.
I responded that I would cooperate and he didn't do anything. I've watched his moods fluctuate.
He was seeming to move towards me a couple months ago and then suddenly became more distant again and has started looking to buy a house where OW can live with him.
We had an open conversation a couple days after he broke that news.
I had to make sure he knew that I love him enough to let him go, but that he still had a chance to put our relationship back together.

I didn't want to have him say "why didn't you tell me?" 20 years down the line. He said he still loves me, but he wasn't happy with me (says it wasn't my fault - seems to recognize it was/is depression) but still says that this is something he needs to do.

I've detached.

I'm okay emotionally and can take care of me and the kids and give him more time, but I'm really scared of the prospect of him buying a house.
He's looking at places that would stretch him to the limits financially and I'm scared that taking on that kind of debt while we're still married has the potential to take me down.

I don't want to file, but I feel like I'd be incredibly naive not to protect myself from that kind of risk. My heart says wait and don't rock the boat, but my head is making a strong case to file for a legal separation.
I don't want to take the move that makes it easier for OW to push him into counter filing for divorce.

Any advice would be welcome.