My ds was really sleepy as well - not helped (or even caused!) by the jaundice he had quite badly when he was couple of days old.
I used to do everything to make him wake up for feeds - undress him, tickle his feet, wet cotton wool balls, even drops of water in his ears! He would still go back to sleep! The little sod even learnt to PRETEND to suck (complete with jaw action all the way up to his ear!) Plus he was really slow feeder, which didn't help. I felt like I was spending my entire life feeding - my bum even got sore from all the sitting!
It sounds like your baby is a reasonably quick feeder, so that is already an advantage.
My ds started off on the 91st centile and then porgressively dropped throught the centiles (although he never actually LOST weight, beyond the inital post birth weight loss), before trundling along just UNDERNEATH the growth charts for a while. He was checked by the consultant paediatrician and eventually we all agreed that he was obviously thriving, so forget about the growth charts as he was following his own unique version! By the time he was 3 months old, he was creeping back into the charts and gradually moved up to about the 25th by the time he was 6months old.
He's now, at nearly three, about average, judging by those around him. I've no idea how much he weighs, but at his 2 year check he was on about the 50th centile.
I got fantastic support from the bf support group at the maternity hospital - run by two of the midwives there, who were also bf counsellors. I got help every week with his latch and eventually we worked out a non-standard "letter box" technique which seemed to work a bit better.
When he was about 2 weeks old and still hadn't regained birth weight (I think that took 6 weeks!), they lent me an electric pump and encouraged me to (first of all) top up after every feed and (later) give every second feed as EBM. That didn't change his rate of weight gain (minimal!), so eventually I stopped bothering - although I maintained the habit of expressing (with an hand pump) as I was going back to work when he was 4 months old, and it also gave me a bit of freedom and allowed dh to share in the feeding.
They also encouraged me to wake him at night, as he was perfectly happy to sleep through, almost from birth! When they suggested every 2 hours, I did every 3, when they extended it to 3 hours, I did every 4, and when then said 4, I stopped bothering! It would sometimes take me over half an hour to get him latched on (per side!), he would then take up to 45 minutes per side (although I'm convinced that he was actually sleeping for part of it). I never got the "proper" signals of a "satiated" baby - he never came off of his own accord - I always had to pull him off - although I took the view that if I could pull him off without "breaking the seal", then he wasn't really latched on anyway.
I went on to bf for over 12 months. That included a skiing holiday away from him when he was 6 months, during which time he was fed on expressed milk and I filled the chalet freezer with sachets of expressed milk (I even expressed on the slopes and once, in bad weather, even IN a mountain restaurant - although dh says I managed to do it so discreetly, he's not sure anyone would even have noticed! )
One thing my Mum told me was how different my brother and I were feeding - she was so worried that my brother wasn't getting enough, as he was only on for 5 minutes (whereas I had been much slower) that she weighed him before and after feeds - and realised he was getting loads! (db was a real porky as a baby!)
I suppose the point of my long ramble is to let you know that you're not alone in struggling and, most importantly to
a) ... ignore the scales. Go by what you dd is "telling" you - both by her nature - is she happy and alert (ie obviously thriving) and by her nappies - does she have plenty of wet nappies? (bf babies don't necessarily have many crappy nappies)
b) ... have confidence in yourself. Yes, you'll forget loads of things and it can takes AGES to get bf established, but take things one day at a time.
c) .... RELAX! Enjoy your baby! She's only this young once!