That's a rough start, and I can certainly sympathsize. DD was a 34 week premie and we started out on the nipple shield for excessive weight loss. We were successful in weaning from the nipple shield, only to deal with excruciatint nipple pain.
In the end, it's going to depend on how important breastfeeding is to you. If you are determined that this is the best thing you can do for your child and that you will succeed no matter what struggles you have, you will most likely succeed. If you think it's too hard, if you think people validating your wanting to quit gives you permission, or that it's not worth the emotional, physical, and mental stress, you most likely won't succeed.
Assuming you want to succeed, feel free to read on. If you don't want to continue feel free to just skip over this one.
There's a huge growth spurt at 3 months and the best thing to do is keep him on the breast as frequently as possible and for as long as possible. It may also be true that because you're using a shield, you need extra stimulation (pumping) to ensure you have an adequate supply.
Can you take a nursing vacation and just hang out on the couch or in bed with the little one, preferably topless and with DS in just a diaper for as much skin to skin contact as possible. There are several ways to wean from the shield. I always offered the bare breast first, whether she latched successfully or not, then nursed with the shield if she didn't. You can also get them started with the shield then once the nursing session is started, pop him off, remove the shield and relatch him.
For the chomping, I would remove him, calmly if possible, and sternly tell him that "we don't bite momma" while making the most serious, angry face you can. Then put the boob away for a minute before starting over. They may not always understand the words, but they can read emotion on your phase and will respond to tone. I've been doing this with DD for the last 6-8 weeks (she's 9 months) for biting because she's teething. Now that her top front teeth are just about through the gums, her desire to chomp down is greatly lessened. If you are consistent, they will learn that biting/chomping down means they lose the boob.
Why did you start offering formula? In order to get your supply up, you must tell your body to make as much as the little one is taking in. For every bottle that you feed, you must pump - and ideally get the same amount he takes in. If you need to use the pump to increase your supply, you should be pumping every 2 hours during the day with at least a double electric pump, but preferably with a hospital grade doule electric.
Anything is possible if you are determined. If you haven't already done so, contact LLL or an IBCLC. Both of these resources should be able to help you address all of the issues you metioned. The WHO recommends exclusive breastfeeding for the first 6 months and continued breastfeeding until age 2 and thereafter as long as desired by mother and child. The AAP, which is expected to revise it's position to be in line with WHO, recommends exclusive breastfeeding for 6 months and then continued breastfeeding with the introduction of complimentary solids for a minimum of 12 months, but preferably as long as desired by both mother and child. There are lasting health benefits for both you and the baby by following these recommendations.
S.