I didn't have a preemie, but had a big problem with forceful letdown and oversupply. I thought we were having latch problems, but he was latching fine...just have troubles with dealing with the flow.
I went to lactation consultant and she had me try a few things. She had me place the baby more vertically on my abdomen while I was reclining back. This means the baby is tummy to tummy with you and fairly upright. The flow then is kind of going downhill (back towards you) instead of straight to the back of her throat while she is lying down. She would eventually kind of lean to the side in the crook of your elbow, but it does make a big difference when the baby isn't totally side lying and forced to gulp the huge downpour.
The second thing she had me try, which was a little harder for me, was this: when you get the letdown, take your 1st two fingers in a "V" and place them on either side of your nipple on your areola and push in when the letdown comes. This should stem the flow a bit, but allow your baby to keep eating. And then you don't have to just let the milk go into a cloth. It was difficult for me to do because i had an oversupply as well and would be so full. So when I'd try this it would pop the nipple out of my son's mouth. So, once I got my oversupply into check a little better, I could use this technique.
here is a site that deals with oversupply just in case you might need some help with that: http://www.lowmilksupply.org/oversupply.shtml
I agree with the others that if she is growing okay, then don't worry about her falling asleep. Since she is more like 2.5 months, that seems to be normal. Good luck!