J.V.
ooooooh this is a tough one. Its so easy to pop that awful I'm-a-horrible-mom hat on. I think what you need to do is visually make a routine and stick it on your fridge. This way you can mentally wrap your head around how long to let her work it out.
Say she wakes up at 7am. She was still pretty young, so we'll say she should probably have 2 good naps a day. Say from 7-9 you have breakfast and play for a bit. When 9am rolls around, change her diaper and lay her down for an hour. Now,if she's not used to it so she'll probably cry. But soothe her the best you can before you lay her down and she may do better than you think. During that time she is safe in her crib. Turn your monitor down so that you can here her if she's in BIG trouble. You need the peace in order to be a good Mom for her while she's awake. Give her an hour for the two of you to rest and try again. She may not do it, but she may sleep longer. If all goes well say 10 or 11am. Get going again. Feed her if she's hungry and play or cuddle a bit. Make sure all her needs are met and then at 12-1pm. put her down again. Same as before, make sure you can hear her, but honor some time for you. When she wakes at 2-3 offer bottle or breast whatever you do. Spend some time playing, errands, whatever until dinner. Say dinner is 5:30 or 6pm. Afterwards start to wind thing down, By 6:30 or 7pm give her a bath (lavender oil drops in the water might help relax her.) I also find massaging my little boy with lavender scented lotion both to be enjoyable and relaxing to him. Read he a story and give her a bottle or breast before bedtime. Lay her down for the night.
Hopefully this will give you some time with your other kids as well as time for yourself. If you get a schedule going like this repeat it for say a week. Babies take a little time to adjust, but I bet after a few days of having a regular schedule, she'll respond by resting better for you. My prayers and best of wishes to you.
Jaime