We can have competing desires, but only one side can "win" in any given situation. In fact, we rarely wholeheartedly do anything. This doesn't mean that we didn't genuinely want to do multiple conflicting things. You're right, if you didn't want to do the drug but did it anyway, you didn't have free will, you felt forced. That's exactly what I'm saying. If you did the thing your higher order desires want, then you feel free, like you really wanted that. That's the experience of free will. You can also say that when you do what you wholeheartedly wanted to, that's free will.

The reason it makes sense to call this free will and not an illusion is because that's what freedom is to a will. Why refer to something that's impossible (going against the laws of reality) as freedom? That seems silly, that's not something we can do and doesn't represent anything in our experience. The way I'm using it is a useful distinction, between when we feel free and when we don't. That's all freedom to a person's will is. Using terms like 'desire' or 'will' aren't helpful here. It's your will and your desires in every case. But only in some cases is your will free.