Sometimes the signs are subtle in the beginning, so small that we doubt them. Later it can progress into fullblown emotional abuse and even gaslighting us so that we doubt our own sanity. Friends and even family fade away for no reason we are told.
Been there, and survived. In my case I do not think my ex had a drug or gambling habit, she was just emotionally handicapped by her own mother, who had been emotionally abused by HER mother.
My ex dealt with it by trying to be the central attraction in her own circus, and spent money profusely to keep the spotlights on herself. For the first few years I thought the "crazy gene" had skipped my ex and passed directly to her daughter. Dramas would pop up that required my immediate resolution or the whole family would melt down.
I have no doubts that I enabled that behavior, it is my nature to want to make things better for the ones I love. Narcissists can easily spot that quality in people and know what buttons to push to manipulate us. I don't know which gender is more likely to be a narcissistic manipulator but once you are aware of the game there is only one solution : save yourself. You must end the relationship as soon as possible! Get away, take your kids out of that toxic environment, and take time to learn what to look out for before starting a new relationship.
Just like in the physical world, the world of online dating has predators and prey. When your senses tell you something does not add up, listen and act.