The problem is, we don't have control over our thoughts either. Just because the happiness is caused internally, doesn't mean it's any easier to change than an external cause. And by telling someone that their problems are all due to their thoughts, it could make them feel like it's all their fault.

Regarding determinism, I do have an article that discusses this: https://edward-john.medium.com/do-we-have-free-will-almost-certainly-not-4ef11c73ebce