Yes sleep deprivation makes EVERYTHING worse! It took quite a few weeks as my DD's top teeth took that long to come through, they were translucent under the gums for literally weeks! I still do get scared of her biting, quite rightly as she did tonight but she kept delatching as she couldn't breathe properly with her snotty nose.
Here it is, I've edited out any identifying info but basically just copied and pasted, some of this really helped me, hope it does you. Hang in there x
"To answer your questions - I did a combination of trying things and riding it out. I learnt to watch out for signs of DS being about to do it so sometimes was able to intercept it, learnt things I could do to quickly release his jaw, tried to discourage (don't think that one worked), so there was some short term helpful stuff but in the long term it was a case of waiting for DS to get over the phase.
First try and identify if you can what the cause for the biting is. Teething is the most common reason, so doing things to ease the symptoms of teething may help - cold things to chew on, teething gel (though use with caution, as if you're using a numbing gel it can also numb the tongue which can cause problems feeding), even pain relief if it's really bad. I know a lot of people swear by amber teething necklaces too, I've never personally used them even though I love the look of them as I'm neurotic about swallowing beads (I think they're very safe but as I say, neurotic!). I found with DS that the biting would come and go as he would be teething, but that there seemed to be other causes that went hand in hand with it. However when the teething overall subsided, the problem subsided with it, so if it is the same for your DD you may find that easing those symptoms may help the problem.
Other common problems are boredom - I was guilty of this one - if you're doing stuff while you're feeding some babies are more inclined to give you an attention seeking bite than if you're cooing and making eye contact - so giving her your undivided attention when feeding as much as is humanly possible can sometimes help prevent it.
Sometimes it's also the case that they're not bothered about feeding. Either they didn't really want to in the first place if you've encouraged the feed and they didn't seem too fussed, in which case try waiting for them to ask for a feed, remembering it's normal for their appetites to wax and wane and if she's not eating as much this week as she was last week it's not something to necessarily worry about. Or more commonly, they'll have the feed they wanted and then keep suckling after having lost interest and give you a good bite "just because they can" at that sort of stage. So if she's doing that, that's where it's really good to start watching closely and notice when she's giving the sign that she's had her fill, and take her off before she gets to that point.
If she ever does it at the point where she's asleep and you're 'pulling out' (suddenly this sounds like sex advice!) then this can trigger an automatic bite reflex, so if this is happening it's always wise to unlatch by breaking the seal with your finger and putting the finger between the nipple and tongue as you come out, to hopefully prevent the reflex and to offer you anti-clampdown protection if she does bite! Same can be said for unlatching even when she's not asleep, you've probably worked this out yourself though anyway!
Also occasionally it can be a latching and/or supply issue, if for some reason she's getting frustrated that she's not draining the breast effectively if something's up with the latch, or if your supply has dropped for any reason. If you're not getting any other indicators about that though then that wouldn't be my first assumption, but something to bear in mind if there are any other red flags. She would most probably be letting you know in other ways as well as the biting if this was the case though.
You've probably already read all the general advice about what to do when she bites. When DS did it, contrary to the advice I would (involuntarily) yelp or occasionally scream (low pain threshold!) which isn't advised as can lead to a nursing strike... you're supposed to stay calm - but I'll leave that one with you. Fortunately with DS that was never a problem and instead of going on strike he would grin in delight at my pain and often come in for a second bite - demon child! (I can laugh about it now...).
If she clamps down with her teeth and doesn't let go again it's a case of prising her off with your finger breaking the seal. What can also be effective if she's clamped down is quickly pulling her face right in to your boob or (very very gently, just enough to close the nostrils) pinch her nose. It sounds cruel but doesn't hurt or bother them, it just means they've got to open their mouths to breathe, and breathing is always even more important to babies than torturing mum! I have heard opposition to that technique, but I can only imagine this comes from those who have never been in that boat personally! Advice I've heard which I WOULDN'T recommend on the other hand is things like flicking the head or even lightly smacking the hand - yeah really - I'm sure that's not something you'd consider anyway but needless to say this can create a bad association and stop them nursing for good, as well as being mean.
And then yeah all the "firmly saying no" stuff which in all honesty never worked with DS at all, he would just laugh at that, but I'm sure it works for some! I would also sometimes try putting him down on the floor after he did it and walking off, to try and create an association between biting and cuddle time ending, but I don't think that ever worked and just ended up upsetting him - but again every baby is different and others may have success with that.
Oh, and a tip for when she gets bigger (if she's not done this already) - the only time DS has bitten me since the phase ended has been when he's approached me for a feed, latched and chomped down and I've pulled out and realised he is still chewing on a piece of apple or something! Ouchy!
I feel your pain, my DD (now 13m) went through a loooooooonnnnnnnnnnnnngggggggggg biting phase. It was horrible. She rarely bites me now but I still have lingering anxiety about it and have to mentally remind myself to relax and put my free hand down by my side to stop myself pulling my boob back. It sounds odd, but I used to pull the boob she was on back so I could check the latch but this would encourage her to bite because I'd be pulling the breast out of her mouth. I found shutting my eyes when she latched on helped (seeing those new teeth going towards my poor - and sometimes healing - nipples was very hard). Also leaving my free hand down by my side so I wasn't tempted to check (my hand still unconsciously wonders up to anxiously poke at my boob and then I spot it and have to tell myself AGAIN to put it down!). At the time I was propping my arm up under a pillow and getting rid of that gave us a better position. Another tip a friend who had a bad biting phase gave me was to really pay attention to her whilst feeding, and holding her in close and snug, and those helped too. I did (and sometimes still do) have a habit of going on my phone with my free hand and I don't think she liked not being paid attention to. If I can find the email my friend sent me, there were lots of great tips in there that really helped me, I'll copy and paste it.