I have constant feelings of doubt in magic. I constantly tell myself that I do believe in it, however I feel that I don't and only think that I do due to fear of Allah. A few days ago I had this thought "If I really do believe can I swear by Allah that it is true?" And I couldn't bring myself to do so. Am I a disbeliever and if I am, how do I start believing?
First of all, Quran is a book of knowledge. There is no doubt in it. That is a firm belief of a true Muslim.
How does one acquire knowledge? Through reasoning. Using God given mental faculties.
Now let's apply reasoning to the world and universe around us. Cause and effect is everywhere. Without a physical cause affecting someone or something is not how nature and physical laws (all made by God - a firm belief of Muslims) work. If it rains, all get wet if u stand in rain - it's not biased towards this group or that group. If you put your hand in fire it will burn. That is a fundamental law of nature that God has made for us.
You may recite 'Ya Razzaq' Dhikr a million times, it doesn't feed your tummy unless you make some efforts to earn your Rizq and pray to Allah SWT that your efforts bear fruit - Example of this is narrated in the Quranic story about Abraham AS and his son Ismail AS who built the Kaaba and prayed to Allah to accept their efforts.
So, in this world where even a leaf doesn't magically move with a cause, it's not rational to think that by some mumbo jumbo someone gains significant unfair advantage over other human beings. This belief if upheld, defeats the whole purpose of creation of human beings which is to test them in this world. This world is a level playing field for all human beings of sane mind when it comes to their evaluation in the Hereafter. Otherwise we will have an argument against God in the Hereafter that Nauzubillah His judgement is unfair because during the test, he didn't keep the playing field equal for all participants.
So, in my opinion, there is no such thing as magic or ability to interact with Jinn - These are false beliefs that have existed amongst humans since time immemorial and it's these beliefs that put humans very close to shirk if not right in it.
First we need to differenciate magic and black magic.
Magic is about doing trick to make people believe that it is real. We can believe in magic because it is pure trick, if you train you can do that yourself.
Black magic is doing real things like cursing others to die, or make loss in their business, etc. like a witch.
You should never believe in black magic. Human are given certain ability, but not so much like killing a person 100 miles away by stabbing a needle into a doll or cursing. It is shirk, only Allah can make or destroy anything from nothing, if you believe a human can do it, you are giving him the power of Allah. Anything that is physically (Physics law) impossible is black magic and believing in it is shirk.
And any working, possible black magic is actually a trick too. If you investigate it and research it well you will find it is a trick. I mean if it is actually a power how come anyone can study and utilize? and some are just belief and myths of people. It will totally oppose any logic and universal law.
There should be no doubt or hesitance in affirming the teachings of Islam. This is definitely an issue which you urgently need to overcome, as true doubt is Kufr. -- But maybe you are ignorant on this topic and knowledge will quell these evil thoughts?
Strengthening your Islamic knowledge and logic/reason is the way to overcome this issue. -- Magic is real. It can affect people physically. Some of the most truthful people (including the Prophet (ﷺ) himself) were afflicted by magic.. experiencing what they were not going through, among other things; they didn't just make these things up. And Ruqyah helped counter symptoms of magic.
Moreover, the Creator who made everything would know what is real or not. Magic is subject to His Will too. He said it is real and that those who engage in it (to hurt others, etc) are disbelievers. Magicians often utilize Jinn, whom we are forbidden to turn to.
Refer to people of knowledge/scholars to help you further. Or feel free to ask more questions here if you have them. May Allah (ﷻ) guide us all and keep us firm on Islam.