Like Dan says there is nothing wrong with you. Don’t blame yourself you didn’t force him to use the stuff.
I can understand the heartbreak and depression, a lot of us have been there with our other halves. We have to get it through to ourselves that it isn’t down to us, we cannot stop them no matter what we do. There is no contest coke will win hands down every time. Lies are part of the course I’m afraid.
Look after yourself and your boys, try to distance yourself from him, his family the devastation it’s causing. Hopefully then the depression will eventually lift.
Keep strong, I wish you well.