Just as how God has made day, He has made night too. Doesn't mean that the night is bad because had there been day for all the 24 hours, man couldn't have rested and other significant imbalances would have occurred. It is worthwhile to note that just like this night and day is the philosophy of good and evil but it is classified in threefold in HINDUISM as follows:- Satva (The absolute truth or the path of truth, only enlightened Yogis who know the function of karma and maya follow it to a 100%. Man functions and works for the benefit of nature and civilization without caring for the fruits of his work. Its like a White color without any shade or taint); Rajas (The path of illusion; most of the worldly people fall in this category. Its the grey zone of spirituality where a man works (does karma) not for the development of nature or civilization but for his own selfish growth and expects fruits of his actions); Tamas (The state of ignorance. The evil doers and the people with demonic traits fall in this purview). Each of these three above-mentioned qualities have their individual traits for e.g. a personal dwelling in Sattvic aspect would have a sattvic diet (healthy vegetarian food (food from plants and milk) only), sattvic vani (courteous language), sattvic vichaar (good thoughts) and since every other thing in his life is Sattvic, his karmas (actions) would also become more and more Sattvic. That is what humans should actually strive to do. These three traits are generated from the energies (or powers of God) or Para Shakti. One is Vidya Shakti and other is Avidya Shakti (maya or material influence). Due to material influence of Maya or ignorance, we tend to possess certain bad habits. If the bad habits over weigh our good ones then we are Tamasic (or demonic) and the vice-versa means we're Rajasic but not Sattvic.
When you say that "There is a devil inside of me" it merely means that the quotient of your material bondage or Maya is so much that it is covering your mind or intellect so much so that you are absolutely unable to feel the presence of the divine transcendental Paramatman (THE SUPER SOUL - GOD). FIRST THING TO REALIZE THAT THERE IS NO SATAN. Satan, Devil, Lucifer, Baal, Beelzebub, the dragon etc. are the terms coming from the Abrahmic religions that are primitive in terms of understanding so their capacity is to divide the positive and negative energies of the creator and explain them as God and Satan. Actually, in Hinduism we call it as Para Shakti (or Supreme Power of God). One is Vidya Shakti (the power of transcendental knowledge) and the other is Avidya Shakti (Maya or the power of ignorance or material blindness that inhibit you to see the divine light). Both these Shaktis combined together represent Para Shakti (means absolute power) and it is wielded by Shaktimaan (the Almighty or Paramatman or God or Creator). So if absolute power rests with God, then the Good and the Evil also is God's creation meaning Avidya Shakti (or what others know as Devil or Satan) is also one of the powers of God and no other entity because apart from God there can be no such entity that can wield powers as much as or even slightly lesser compared to HIM to defy HIS WILL & defeat HIS PURPOSE. Its not possible. God is supreme consciousness and no one can defeat His purpose and that is how we define God in Hinduism. He doesn't demand your faith, He doesn't demand your sacrifice, He doesn't demand your knowledge & what's more, He isn't going to judge you on the "JUDGEMENT DAY" also. He maintains a dynamic log of our karmic actions so He has already decided where each soul would go afterlife. God just wants you to understand how the Prakriti emanates from Him and how beautiful it is in all its glory and splendor. He is a part of our Soul if we dig in deep into our self without trying to find Him in cosmic horizons. He wants us to have a really beautiful experience of nature first hand. Be it night and day, life is like a virtual reality video game where you live, eat, drink, fight, bruise, bleed, die and work in all its glory and grandeur. This game is going to be over one day (because you can't keep playing games all the time) and you will return to realm of reality and realize your true oneness in God on spiritual planes. If you realize this fact and try to experience God in whichever way possible (best is Yoga), then all the material bondages of Avidya and darkness (demonic forces or devil or satan) would vanish from your mind. Constantly reminding yourself about the creator undermines His very own creation amongst which we get lost a lot of times in our life due to the sheer amount of vastness in his creation in ever sphere, in every dimension, in every space and in multitudes of different aspects. The problem is that we're so much fascinated by the creation that we've forgotten that there is a creator to it too. Even in biblical tales, lucifer was God's angel created by none other than God so why do you pay so much attention to him and his attributes while the creator goes without any sufficient or adequate credits???!! We worship Shakti but we should actually worship the Shaktimaan with whom the Shakti comes along. That my friend is your solution. Hope this answers your question. Whenever you feel that ignorance or material desires are trying to bind you, try to think of God first and then death which is the ultimate reality that would remind you about WHO RUNS THE SHOW OUT HERE!!!
Om Namah Shivay!!! Om Shanti Shanti Shantihi!!!