Is the statement "That's it, I've decided," uttered with the intention of dissolving the marriage contract due to obsessive-compulsive disorder, considered a divorce, given that the marriage has already taken place three years ago?

The aforementioned phrase does not constitute a divorce. It is either a promise to فسخ (annul/dissolve the marriage) or an announcement of an intention to do so. Divorce does not occur if it is driven by obsessive thoughts (waswasah) and not by choice. Therefore, there is no justification for fearing that the wife has become unlawful, and these obsessive thoughts should be disregarded.