My experience with mental illness is that it simply does seem to flare up at exactly the wrong time. I never can exactly figure out why I can be doing really well for a long time and then poof, I'm not. The thing that is tough for me to remember is that it always passes.

I guess what I would hope for you is that you would be patient with yourself, and try to do the least harmful thing that will still work to get you through this. It isn't that you have failed. I bet you are actually doing much better than you would have if you had not learned the DBT skills at all. This is just the nature of mental illness. Hopefully the times that it happens will get less and less. But it's like diabetes. We'll probably have to keep an eye on it for the rest of our lives.