HI K.
I am replying mostly to tell you I can commiserate with you but dont' have any better answers than what you are doing! I have my 2nd daughter who is 19 weeks, my first is 22 mos and I had the same issue with both of them. My oldest didnt' sleep through the night til she was 9 mos old and that is when she stopped nursing because I got pregnant (they don't like the taste of the milk they say). My baby is still nursing several times at night, I usually get one 3-4 hr stretch then she has to co sleep and be able to just turn over and nurse or she is up every hour the rest of the night.
My philosophy is they all get bigger and grow out of it at their own pace. I tried every method with my oldest daughter and it really came down to when she was ready she did it. Now she is back to waking up and sleeping in bed with us again !
The one thing that really helped get them both down is a consistent night routine, bath, books, nurse, bed-sounds like you are doing that. Both my babies I choose to put to sleep on their tummies as that is the ONLY way they would sleep on their own. I can also relate that I can't let my baby cry as she wakes up her sister, but I also can't handle the cry it out method, I just don't believe in it.
So my thoughts are with you , but hang in there and it will work, just be consistent! Someone told me about a book called 12 hrs in 12 weeks ( a dad of triplets) he said it worked for them, I haven't read it though.
Good luck!