Its said that negative belief creates blockages or causes bad magickal effects. Lets say you have a doubt or fear of something. If that already manifests in your mind does it mean you have to do something to 'fix the damage' or would simply forgetting it entirely do so already? How does it work?

If a negative feelings/belief manifests does it mean the 'damage has already been done' as a simpler way of saying and thus you have to take care to dodge all of it?

Beliefs are a choice. Feelings are harder to change.

If you hear a loud noise, you flinch, right? Well, how do you stop yourself from instinctively flinching? You have a momentary "fear" even if it isn't something you're doing consciously. That doesn't make it bad--it's a survival instinct.

Would you call that "damage"? I sure don't.

On the other hand, thinking "my religion = the only right one" is a belief, and could lead to interpersonal problems when you butt up against someone who doesn't share the same views. How would you go about changing that belief? Would you study up on other religions to get a better understanding of what they have to offer? What if you found something that resonated more as "truth" to you than what you had been brought up to believe? Would you deny that you'd ever seen it, try to forget it--or would you consider changing your fixed idea?

If you're doing a magickal work, and you've got doubts in the back of your mind--when the point of the process is infusing your own belief and will into the spell manifesting the way you desire--do you really think it's going to work the way you intended when your focus is on all the ways it could go wrong? It's (usually) important to have a clear and focused mind when it comes to metaphysical workings if you intend to have any success in what you aim to accomplish.

You may want to look up and study the subject of shadow work (the process of learning how to identify and heal your hidden, painful, self-sabotaging beliefs) since that seems to be the topic you're dancing around here.