Like real people, characters should have flaws. After all, Nobody is perfect and your characters need to be Somebody. Hence, they need to have flaws and negative traits to help balance out their strengths. Having a negative trait in their character also helps provide conflict and gives you as the writer a place to build their arc.
Take a look at your character’s negative flaws and ask questions. Are they quick-tempered? Stingy? Vain? Perhaps they have low self-esteem or they care too much about what others think. Once you know where their shortcomings are, ask yourself how it impacts their ability to resolve the main conflict. Does an inability to listen to others cause a miscommunication? Does their timidity cause them to keep quiet when they have a perfect solution?
Their flaws should impact them in some way. Use those negative traits as an obstacle to getting what they want. This forces your character into needing to make a change and gives them a motivation for their character arc.
As the arc progresses, ramp up the problems caused by that negative trait. As the results become worse and worse, your character is forced to try a new tactic to get what they want. This reinforces the idea that their negative trait needs to change.
Not every character arc will end with a complete turn around. Change is hard to do, especially when it’s something like a bad habit or a negative trait. Rather than forcing your character through a full reimagining by the end of the story, let that negative trait remain—but tone it back. Show they can still be just as stubborn, stingy or selfish as they were, but that their instances of doing so are lessened by the impact of their past actions.