I feel your pian. My son (now close to 11 months) was the same way. Although he was not a preemie (10 days late, actually), he was a constant nurser. In the beginning months (read: until about 6 months), I found it enormously tiring. But, I reasoned with myself that if he didn't need it, he wouldn't be doing it. And it is so natural for a child, that I felt guilty not giving it to him. One tip from my LC was when you releas the suction, try to immediately close her mouth so when she tries to suck, the pressure increases again. That worked well for me about 1/2 the time. Sometimes I would have to hold his chin up gently for a few seconds, but it is something you might want to try.
Also, if you are comfortable, try laying down to nurse. I found that it relieved the pressure of bad posture and such. I didn't try it til much later and regret it.
One more side note: don't worry about supply. If she's on you all day, you'll have more than enough.