Oh, my. I feel your pain, I have been there!
My oldest son was born at 36 1/2 weeks. Not the earliest of preemies but he was on the small side and had his share of troubles. The worst was feeding.
He was born with a severe tongue tie and a short tongue, and was unable to nurse. His tongue was so short, and pinned down so far that he wasn't able to get it up under the nipple to 'work' the breast. The lactation consultant and I tried everything we could think of to try to get him to latch, but to no avail.
To further frustrate matters, rather than fix the problem (clip his tongue) the NICU gave him a 'special' bottle called a Haberman feeder. (It's designed for babies with cleft palate, etc. ) Granted, it was better than seeing him fed through a tube (which they threatened to do) but I still had a very tough time getting him to latch on when I BF him. So, I too, was told to use a nipple shield. It helped a little bit, but only when we got his tongue clipped did he really do better with the shield.
Like you, I thought the bottle had caused him to become 'lazy' when nursing. He fell asleep at the breast EVERY time. I'd do everything I could to wake him up and get him to eat more, but I could never keep him awake for more than 10 minutes max. I was told by the hospital that the reason he was falling asleep is that he had to work really hard at the breast, even with the shield and that he was tiring out. They said preemies tire out alot quicker than full term babies do and just nurse him more often, rather than try to prolong his sessions. I did just that and it worked out--he seemed to be less stressed when nursing.
Once he was home, we settled into a routine and I continued to use both the 'special' bottle and nurse with the shield, but over the first 2-3 weeks I decreased the use of the bottle gradually, then enliminated it altogether. Then once he was only nursing, I gradually decreased the use of the shield. I would let him get almost done with a feeding, where he was good and happy, and I would take the shield off and let him try just the breast for the last few minutes of the nursing session. Then, next session, I'd take the shield off just a bit earlier, and so on and so on. I did this little by little until he was just a little over a month, almost a month and a half old. It worked! By the time he was a month and a half, he was nursing with no nipple shield and doing great. Mind you, it wasn't easy and there was many a time when I wanted to just give up, but I stuck with it and I'm glad I did.
So, I hope this helps you. I know that every baby is different, and things don't always work the same for everyone, but maybe it will help. I wish you lots of luck and I'm glad you're giving your preemie the best thing you can give. I hope he comes home soon!