I think it's harder in the summer since it stays light out later. Plus, as others have said, you have to take into consideration her activity level and nap vs no nap.
I've always marveled at parents who have kids who go down at 7pm. My girls have NEVER gone down early. My just turned 3 yr old wakes up somewhere between 6am and 7am. Her normal bedtime is around 8pm, but right now she goes to sleep somewhere between 9pm and 10pm (occasionally later, but that's if she has a late nap).
You do NOT have to lay with her until she falls asleep. What we have done is tell our daughter that she does not have to go to sleep, but she does have to lay in her bed and be quiet. She can read books, play quietly with her stuffed animals, etc, but she must stay in her bed. We leave a night light on, turn on her "sleepy" music, and close the door. She is allowed to come out to go potty, and that's it. She feels "empowered" to go to bed when she wants, and we aren't "fighting" with her about her bedtime.