Modestmothering, I promise, it will get easier! What a great sacrifice you are making for your little angel.
There are some good tips here... I definitely second the LLL support. They helped me through 2 or 3 crisises during our BF journey. I thought my DD was self-weaning somewhere around 8-10 mos (can't remember) but learned it was just a nursing strike. It lasted a day or two! I couldn't believe it at the time but it was true. The woman told me to just keep offering the breast every 1/2 hour or so which I did, and low and behold she finally started nursing as if nothing ever happened. So it's true, babies don't self-wean. BUT if a mother *needs* to stop weaning, many will decide that the nursing strike is the right time to stop nursing altogether.
Teaching manners is vital. Think about it in terms of, will this behavior be acceptable when she's 10 mos? Or 1 1/2 yrs old? If not then it's better to nip it in the bud. A LLL leader will be able to help you address how to correct specific issues and also put you in touch with your closest LLL group meetings for support.
I also agree with eating snacks throughout the day and especially before bedtime -- something substantial like a pb & j or other sandwich, cheese and crackers, bowl of leftover pasta & meatballs, slice of pizza. If you're not vegan or lactose intolerant I always found a 1/2 glass of milk would help top off that full feeling. I remember eating A LOT while nursing... more than when I was pregnant.
BTW, I do remember during growth spurts (and your 5-mo-old is most likely having one around now) nursing DD up to at least 5x an hour. Maybe even more some days. I know it sounds extreme but when she fussed I stuck her on the breast. So literally every 10-15 mins while she was awake for a stretch of days. Then it will pass and be less for a while, then repeat in a few weeks when another growth spurt comes along. If you don't work (or work at home) I would suggest, if you don't already, that you wear her almost constantly in some kind of carrier or sling. As soon as she fusses, nurse her. She may not be as ravenous when you do nurse her if she's eating til her heart (stomach's?) content. I'm sure you know but it's important to remind yourself that babies nurse for comfort, too, not just nourishment.
Hope you get the help and support you need. I'm sorry your experience right now is not bringing you as much happiness as it could be. You sound so smart and determined, it makes me glad for your little girl and for you. I will be interested to hear in how things progress and work out for you! Best wishes.