Dear S.,
I'm working through the same things with my son, but he's 4.5 months old and I've been back to work since month 3. I'm a first time mom so I don't have a wealth of experience. I'll just tell you what it's been like for us. When I went back to work, my husband was still on leave. My son, whom I've breastfed, was sleeping in 2 hour stretches at that time, with occasional 3-4 hour stints. He didn't have regular nap times, but bedtime was fairly constant. He slept in an Arms Reach Cosleeper when I was awake enough to put him back in it after a feeding, with me when I fell asleep right after a feeding.
We put our son in a very good family daycare for ~6 hours a day when I went back to work. He seemed fine and his sleep cycle was unchanged at 2 hours. Then, 2 weeks later, my husband went back to work full time and our son was in care for 8-10 hours a day. He started waking every hour through the night and his naps at daycare got shorter and shorter as the week passed, down to none on Fridays. After 2 weeks of hourly wakings I was a mess and he was very obviously stressed. I took a week off work and reduced our son's daycare time to 6-7 hours a day (I spent those hours sleeping and trying to keep up with my email at work). I also read the "No Cry Sleep Solution" by Pantley and concentrated on regularizing naps on the weekends and getting my nipple out of my son's mouth before he dropped off to sleep. His sleep cycle extended back to 2 hour cycles. I went back to work, but cut my hours to 6-7 so that he wasn't in care for more than 8 hours a day. He stayed at 2 hour cycles which was not enough sleep for me.
By this time he was 4 months old and over 16 pounds. Our pediatrician said he could easily sleep for 4-8 hours at a stretch and gently suggested that we let him cry it out. We tried, but my husband caved after listening to both my son and I cry for 40 minutes. Then his first cycle of sleep, when we put him down at 7:00PM so we had some time to ourselves to eat dinner, started to degrade. He would wake after 10, 20 or 30 minutes and cry for me. I'd go to him, nurse him down again and put him back in the cosleeper. Sometimes he'd wake again and I'd have to nurse him down again. It began to seem like he just didn't want to sleep without me next to him. I wondered if I should just give up on the cosleeper and keep him in the king size with me. However, I'd observed all along that he slept longer when he slept alone. Then one night we had a friend over for dinner who happened to be a pediatrician. My son did his usual of waking up 20 minutes after I put him to bed. My friend talked me down through 45 minutes of listening to my son's cries cycling up and down from fussing to frantic and back again. Finally he fell asleep and slept for 2 hours. We began to let him cry at bedtime and he never cried again for more than 30 minutes since that first night. Most nights he fusses quietly for less than 5.
However, he was still sleeping for only 2 hours at a time and I was feeling more and more exhausted. Last week I began the arduous task of letting him cry in the middle of the night and not picking him up until 2.75 hours had passed. Again I had to drastically reduce my work hours to have the stamina to be consistent all night long. One night he cried for 50 minutes and it was sheer hell to lay in the room with him and listen without picking him up. I was drenched in sweat. Every time I stirred, he'd stop crying and wait for me to pick him up and then resume with even more vigor when I didn't. Very, very difficult to get through. However, last night, for the first time, he made it 3 hours between each feeding, without any crying.
I would have vastly preferred not to have let my son cry. I had a tough childhood and was never allowed to go to my parents for help in the night, so I hate not responding to my son. However, at 3-4 months old, there are only so many soothing measures you can offer. Reading stories doesn't help. Singing was of limited use. Rocking never worked with my son. Walking through the neighborhood with him in a carrier did, but I wasn't up for that in the middle of the night. My husband's job is too demanding for me to shift the responsibility to him part of the time. Crying it out is what has worked and it has worked quite fast. My son seems to "get it". He changes his behavior immediately. I have to be careful to be consistent or he will backslide, but we do make solid progress. At first I couldn't go in to reassure him without causing him to cry even more. Now I can go in and check to make sure his blankets are still in place, stroke his head and it seems to help calm him and shorten his fussing.
Good luck with your sweet little girl. Every child has different sleep needs and you will eventually find what works for you.