In a baby, sucking the fingers is a "hunger" cue. One of several. She must be hungry.
You need to feed on-demand... not on a schedule, if that is what you are trying. Putting a baby on a "schedule" for feeding, is not good for a baby because it does not take into consideration a baby's own nutrition demands or growth spurts and their increasing needs for intake, daily. THus, many babies who are on feeding schedules, sometimes do not develop/grow/thrive as well. Because, a baby's intake needs, can vary EACH day. They are constantly changing. But, a "feeding schedule" does NOT change with the baby ... it is static. A baby is not static.
Your baby may be going through "cluster feeding" as well... and this is normal. It means they feed even every single hour. And they need to. It is a biological intake need.
When I had my first baby, I got the Baby Whisperer book too & know it very well. BUT, I used it as a reference... and I learned from it what a baby's "cues" are, and what their cries mean etc. BUT, each child is different... some will simply NOT handle a "schedule" or strict routine. Other babies will. So use it with discretion. I did not follow it... but used it to understand my own baby and I did my own thing.
MANY babies cannot sleep, per all the developmental things they go through.
Nursing a baby, cannot only be done when it is "activity" time, per the "EASY" method. A baby needs to feed, when they need to feed. On demand. I think, your baby is simply hungry and needs more intake, and more frequently.
OFTEN times, a baby will not/cannot sleep, because they are hungry. Is she latching on properly? Is your milk output adequate? If not, then she is probably NOT getting enough intake, and so she is basically hungry all the time.
Now, when a baby nurses, it naturally makes them relaxed and sleepy... and so, most babies fall asleep at the breast. But we have to feed them, right? And yes, it pacifies them. But, she is only a baby and it's a natural instinct. It will pass.
But it takes time.
There are 2 approaches: (1) going according to your babies needs & cues and time-lines, or, (2) using a method and this may or may not work, thus frustration for both Mom and baby.
I have 2 kids. Both of them are each different from the time they were born. I could NOT have used a "method" with them, because they are each different. I had to cue into my child, and instinctively, adapt to THEIR needs. Yes, it's not easy, its demanding, they wake at all times of the night, each month they change and are different, NOTHING is ever static, a baby constantly changes, and I have had to change with each juncture of age-stage they went through. Despite my NOT having any sleep either. Oh well.
I guess my advice is, don't worry. Use your instincts and your baby's cues to help her. You have NOT 'created a monster'... ALMOST ALL babies are this way. That is what a baby is. Naturally. And if she needs to feed, she needs to feed. Simple.
But, for me, what I did do, is have constancy, regularity, and a stable routine, EVERYDAY, for my children. From about 6 months old, I got to know THEIR patterns very well... and I went according to that. I did not try and put them into "my" schedule, but theirs. My son does very well with routine & is a good self-soother, my daughter was not & was very high-needs and clingy. BUT, they both had regularity as far as feedings, on demand, and comforting, and naps, and sleep times. I went to them and comforted no matter what. They are now both very independent and affectionate children.
I know, each Mom is different... but for me, this is what I did.
I think your baby is just plain hungry and the EASY method is not suiting her. She gets frustrated sucking her fingers because no milk is coming out.
Kids will have all sorts of "habits" that adults feel they have to break later. But that is later. It cannot be avoided completely. If they need to soothe, then they need to soothe, somehow. They will not go to college being at the breast or need your breast to sleep. All in due time... things happen, when it is naturally time.
For me and when my kids were babies, I had a general routine of: -they wake-up, I nurse them, they are awake, I nurse on demand and before they napped/went to bed, and nursed on demand at night if they woke. Again, I knew when they were tired. A baby naturally gets "tired" about 2-3 hours after waking. An "activity" does not mean constant playing for a baby... it is just being awake and normal natural things you do with them and even a bath is 'awake" time. And its NORMAL to nurse before a nap or bedtime... and a baby is naturally tired/sleepy at this time, so it just so happens that they fall asleep while nursing. It's not necessarily a "bad" thing or a "bad" habit.
Sorry for rambling... I didn't mean to.
take care,
Susan