Just use the 6R’s. It is only one of the five hindrances. Doubt is just a type of thought.
Can you observe it and then let it go? It isn’t yours – you are identifying with it as my doubt.
The more you apply the 6R’s, the more doubt will start to weaken. You stop feeding it, so to speak. In time you will start to notice that this practice is very effective and doubt will just fade away.