N.-
I sympathize!! My daughter (who was my first and is now 2.5 YO) pulled the same trick on us. I was returning to work and Daddy was taking the weekday "shifts" so there was no choice but a bottle. Our perference was bottle-fed BM, but she needed to take what was available.
My fisrt week back was hellish. I ran home early for three days straight to nurse her as she was going more than 6 hours! By the time I would get home she was a crying whiney straving graveling mess. It tore at my heart. On day 4, Daddy took her to the doctor. She was checked out and told that it was a preference issue. The doctor's orders were to go home and basically suck it up. It meant we had to "break her" and that it had become a matter of wills. It also meant I had to pump constantly at what would have been every feeding. We were told that she had to be feed by a bottle for a minimum of 3 days, preferably 5 days and that when we did reintroduce the breast that we needed to be random about when she got it (i.e. once at breakfast then the next day at lunch so she wasn't always expecting to nurse for breakfast).
The poor little thing went over 14 hours before she took a single ounce and collapsed from exhaustion. She slowly took a little more during her night feedings but I knew it wasn't the amount she was used to (not when I was pumping six to 8 ounces!!) Daddy and I took shifts as the crying shook me to my core and tore at my heart. I usually left when he was trying to feed/console her. When Daddy left, I usually wore one of his shirts so I would smell less like myself.
Ultimately, it worked. After the first 3 days, she took the bottle like a champ. We stayed at it for the full 5 days as recommended then started her on the breast again at random times when I was home. This took a lot of commitment and if you have the support, get other people involved. It was tough for just the two of us and I would have loved having the grandparents around to help.
After about 2 weeks, she was able to nurse from the breast or a bottle without a care. She would flip back and forth all the time, but I think she really liked when she got to nurse when I came in the door at night at 6 pm. She knew it was "Mommy time" and always waited for me. As she got older and could crawl then toddle, she would coming "running" as fast as she could as soon as that back door creaked! LOL.
Looking back, all I can think about is how willfull she might be as a teenager! :-) It was totally worth it though and we continued to nurse, evenings, nights and weekends, on demand until she was about 15 months. It was really our special time together and I think she is healthier for all that time spent.
If you are serious about BFing, it should not matter the delivery method of the milk. Don't give up. Be persistent. You are the Momma and you make the rules.
Good luck.
~C.