I have a 15 month old daughter who slept great when she was 3 and 4 months old, but never again after that. She's never had a pacifier and she has her own crib, but it's in our room. Our current living conditions prevented me from teaching her good sleeping habits. Me, my husband, my four year old daughter, and the baby all sleep in one room together. I tried letting her cry it out, but all the noise gets my other daughter up and it's harder that way. I tried a soother but she always rejected it. I tried a comfort toy but that didn't work as well. She weighs 21 lbs but we still rock her in our arms because that's the only way she'll fall asleep. She wakes up every hour or two, sometimes even every half an hour, expecting to be rocked again. And we do, otherwise she screams violently and then wake the other one up. My husband and I go to work at 5am and sometimes I go with very little or no sleep, and it's very disruptive to our lives. I realize that these problems are all my fault but I honestly don't know what to do anymore. Help please.
I think you will suffer much less if you find a way to change your child's sleep association as soon as possible. You deserve to be well-rested, and it will help your relationship to your child too. Kids brains are incredibly flexible because of all the neurons. Right now she has a habit that is working very well for her to get back to sleep and not well at all for you. If you trust her and use a gentle, firm, consistent method, she will learn a new habit and you will feel so much better.
I suggest the book, Solve Your Child's Sleep Problems, by Dr. Ferber. I found his writing very well-reasoned, and his method very detailed so I knew what to do. It worked to change my son from his association with rocking and nursing back to sleep, and it took three days before I went from getting 1-2 hours of sleep in a row at night to getting 7-8 hours. Afterward, my son's schedule was going to bed at 7:30 or 8pm and getting up at 7am.
I know it is hard to do any project when you are tired, but it is so worthwhile to have the results. I suggest you read the book through and take some notes, and plan your strategy. Pick a time when you can either bear to be extra tired for 3-5 days or else have some help around the house, or both. Ask a friend or parent to do this with you if you can, ask your partner to take some sick/vacation days if you can so he can help out, and/or ask a relative to take the other little one so at least someone is getting good sleep.
After I read the book and made my plan, I did Ferber's method with my mom in shifts, while my son's more soft-hearted dad was out of town. We wrote down "the rules" we wanted to follow for sleep training and kept a log about how it was going, because it is very hard to stick to it, but also very important. When it was my mom's shift I slept for four hours in a row with earplugs in, and even though I was upset about sleep training and hearing my son cry, the extra sleep felt like heaven because I was so exhausted from waking up every hour or two to rock my son back to sleep.
The first night is the hardest, and it gets a little better each night after that. I love my son and can't bear to hear him cry, and I toughened myself and sleep trained him because I knew it would give our family the best life together over the long term. It helps while you are doing it to remember that if it isn't better after 5 days, you can always try something else.
Children associate things with sleep, with my daughter it was drinking milk. It took a couple of nights of letting her pitch a fit before she unlearned the association. My daughter is 2 1/2, and it took us 2-3 nights. It might be a little more difficult with a 15-month-old.
The way I see it is that you have a couple of options:
Regardless of what you choose, both options are going to lead to more sleepless nights before they lead to fewer. If you go with option one, you will probably battle the 4-year-old getting out of her bed and wanting to come get into bed with you for a few nights and then after that you still have to sleep train your younger daughter. On the plus side, once she got accustomed to sleeping in her own room she wouldn't be disturbed by your other daughter crying in the middle of the night while you sleep train her. For me, having a non-cranky four-year-old might make it worth the effort. With option 2, you cut down on the amount of time you personally are going to lose sleep (most kids I know that respond to CIO are sleeping better by the end of about a week), but everyone in the house is going to be pretty miserable--including your 4-year-old.
Whatever you choose to do, both you and your husband are going to have to be on board because you're going to have to work together to do get it done.
Good luck to you! It's so tough, but it will get better.