igmum2b, I'm sorry you are anxious and have had a tricky start.
What no one tells you in advance about BF-ing is how often new babies feed. They feed, and feed, and feed, and it is exhausting, but it doesn't mean anything is wrong with them. The reverse, in fact.
At 3 weeks, my DS was glued to me, and tbh, during the day, I might get one break of about 2 hours, but otherwise he was feeding on and off constantly. On the boob for an hour, 40 mins off, 10 mins on, 20 mins off, back on for an hour...
I found in the end it was best to stop thinking about 'feeds' as timed events, with a beginning, middle and end, which would leave the baby full until their next 'feed.' That's more of a formula feeding mentality - it's not how most BF babies work.
If you genuinely want to to keep BF-ing, I'd say go with the flow, let your baby feed as much and as often as they want, and yes, it is SO tiring at this stage. And the nights are a killer (again, not much about that in all the pro-BF-ing pamphlets the MW gave me...)
If you can get to 6 weeks, it gets easier. After that, easier still.
When it feels as if your breasts are empty, they still produce milk (I remember wondering about this). What it does mean is that your DD will probably need to feed for longer to get the amount of milk she wants. It's only really a couple of times a day you find yourself feeding a baby with very 'full' boobs - the rest of the time, they're making it as your baby is feeding.
And like everyone says, this is how a baby orders up the amount they want, espcially during growth spurts (which are all over the 1st 6 weeks...). So yes, you do feel like your breasts aren't full, and if your DD is feeding very often or for long periods, you wonder if it's because she isn't getting enough. I think that's a very normal experience, but so many BF-ing mums think there's something wrong because of it.
I hope you can get some good advice - GPs vary so much IME in what they know about BF-ing. A BF support group might be helpful - your MW should know if there is one near you.
Good luck.