He says he believes that if there was a bad middah of his which caused his anxiety, it would likely have been gaava. He believes he had been setting expectations for himself which were unrealistic (at least in the short term, maybe also in the long-term) and the root of that mistake was gaava.
He also cautioned that for other people, it could be an excess of anivus which causes anxiety; and that just improving other middos isn’t a complete solution, since anxiety itself can be a negative middah which must be worked on with its own tools.
i don’t think it results from a bitachon deficiency, but it could be that taking a daily dose of bitachon over a period of many years could aid in achieving wellness. I know someone with anxiety very well and trust me he has no less bitachon than anyone else.