Doing a spell/working is much like building a tower from blocks. Not just a small tower, but a monolithic tower. The blocks are you, the Beings you may be employing in the spell, your preperation, your dedication, your Will and Desire, your ritual tools and the list goes on and on. Now, in building a tower - you must have enough self-confidence and belief in yourself in order to make this tower not topple to the ground. This is the purpose of this writing - self-confidence and belief in yourself.
Since birth in this world, you are taught to be in a certain way, think a certain way, believe in certain things etc - so unless you were born to parents who encouraged your Occult Path since you were a child - you will be doctrinated under what society deems important and not important. You can see how this will effect you in the Occult Arts (not just concerning external Forces such as Beings etc) - but with you! Belief in yourself, self-confidence! You are taught to have self-confidence, yes - but not to this extent and degree. The amount of self-confidence you must have in order to get a job at a company is much less than what you need in the Occult as an example. You need to have so much confidence and belief in yourself and your abilities - that you will not wonder about the working you have performed...as you will KNOW that it will be as you planned it.
Before, during and after any working or spell - you must not once doubt what you are planning to do, what you are doing, and what you have done. This does not just include doubting the Beings/Forces you have worked with - but you. The Forces you work with just assist you to reach your goal if you cannot do so yourself at that point in time (not unlike a shovel helps a grave digger to dig a grave) - so regardless of Them - it starts with you! How can you expect to even trust an external Force enough to assist you if you do not even trust in your own abilities?
Self-confidence and belief in yourself means that you will know where you currently stand in your path of spiritual growth - what you can and cannot successfully perform - if you can do a working yourself or if you would require an external Force - if you can push yourself further or not - if your reason for persuing the spell/working is even worth your effort or not and much more!
This actually also counts in Evocation - Calling forth a Being. If you do not have self-confidence, the Being/Spirit you Evoke will know this - and not all Beings/Spirits will purposely overlook this fact at all. Many of Them will exploit this fact and you will end up worshipping a Being/Spirit unknowingly - and this is bad. (Do not worship any Being/Spirit. Remember - you are a pracitioner, the magick you perform stems from you and your Will and Desire, Beings/Spirits are simply there to assist you to grow spiritually until you do not need Them to help you any longer. Worshipping something will limit you spiritually, and you will not grow spiritually...as in all that worship, all that energy is directed at the Being/Spirit and not to you where it is most needed for growth. Worshipping a Being will also cause you to feel "fine" in your spiritual stagnantation, that you will not develop yourself as you must, because you feel no need to do so. Remember - working with a Being/Spirit is different from worshipping Them.)
Also remember that not all workings are the same, as not all practitioners are the same. This means that just because a ritual worked for one person, it might not work the same for another. Even with Beings/Spirits - as an example, just because John Smith who practices the LHP worked well and successfully with Lord Belial, it doesn't mean that Jane Doe who practices Neo-Paganism will experience the same with Lord Belial at all.
Negative self-doubt, insecurities, unrealistic expectations etc are some of the pracitioner's greatest enemies - and as all greatest enemies, they are not some foreign external thing...but they are you. Conquering yourself is the most difficult thing you will have to do in your path of spiritual growth - and it is a path that all of us undertake.
On closing - here's a quick note on spiritual growth: Even if you do not consciously consider spiritual growth and development as one of your ultimate goals in your path and practices - with every act you do in your path practicing the Occult Arts, you do develop and grow spiritually.