She's still 7. Of course she needs reminding, and of course she gets side tracked. Let's not jump to any conclusions about ADD, please!
When she turns 8, tell her that she will be responsible for remembering ________ (fill in the blank). Don't give her too many tasks/chores yet. But at 8, she should have about five. Give her a chart to hang on the wall to mark off when she has done whatever she is to do. You may or may not want to remind her once a day to check her chart. Then, when you are heading out to do something she enjoys (going to the park, shopping, a friend's house, whatever) look at the chart and say, "Oh, I see you didn't ________. We won't be able to go now because you need to do that." This should help her learn quickly to pay attention to her chart and what she should be doing.
If you are asking her to gather her homework/lunch/instrument because she'll need it for school and next thing you know she's heading out the door without it, I say let her forget it a couple of times. When she gets in trouble for not having her homework, or is hungry because she doesn't have her lunch, she will be much more likely to take responsibility for remembering it.
No need to get mad, anymore. Just let her live with the consequences of her actions.
My 8 1/2 year old does not forget things anymore because she has already been through this lesson. : )
Good luck!