Doubt can have many meanings.

Actually, it depends on the situation and circumstances. When your father told you this, you were very young and didn't have the ability to understand the depth of his words. Maybe you took it negatively. But now that you're at a stage in life where you can understand things better, you'll realize that your father was right.

I firmly believe that parents never think ill of their children nor do they intentionally teach them bad things. Whatever they said at that time was based on their own life experiences and bitter truths, so that you wouldn't have to face the same difficulties that they might have gone through.