I had to get my magic stronger, this doesn't work in the sense of you have more magic or that you can run out of magic, mind you. Magic is magic. The closest comparison you can give would be a muscle mixed with music. Everyone born with magic can use it at the same level. An eleven-year-old child could theoretically have created the wards around Hogwarts if all it took was Magic since power levels of magic don't exist. The factors that determine what you can cast and how much you can cast come down to how much you handle magic and how often, If you consistently strain yourself with high-level spells or complicated exercises that deal with control, you'll soon find you can cast more and for longer. Why is this you ask? Well, it's to do with how in tune with your magic you are. Both over time and as you cast more and more straining spells that require control you get more accustomed to Willing your magic into certain actions. the more time you spend with your Magic the easier it becomes to handle. This is why older Wizards that fought in wars are so powerful (See: Dumbledore). They have spent so much time casting highly straining spells that require fine-tune control that it becomes easy to use magic for almost anything.