You Feel Isolated and Alone
You feel like you are walking on eggshells; the smallest things will make your partner lose their temper. If your friends ask you to come over or if your family asks you out to dinner, you're scared to tell your partner for fear of their reaction. Your partner wants to know who you're friends with, making you feel guilty about spending time with them, and eventually you slowly start to spend less time with your friends until you feel like your partner is the only person left in your life.
They learn about your vulnerabilities and weaknesses so they know where and how they can hurt you, and cause you more pain. They make you feel alone, confused, scared, embarrassed, or worthless until you no longer have any confidence left in you. All of this is so they can make you feel isolated and lose all of your support, giving them more power and control.