Without hay a lot of rabbits develop overgrown molars. It's quite common but usually not extreme enough to cause major issues. Trimming them can be expensive and needs to be done by a competent vet not a dog/cat vet. It requires at least light sedation and special tools to get to the molars and trim them up. Quite commonly done on pet rabbit and guinea pigs where teeth issues are watched for more and they are fed a wider variety. Pellets aren't too hard to chew up which is part of why molars get overgrown but it also causes the overgrown molars not to cause problems. Pop a pellet in your mouth and it will mostly turn to dust by moisture. Only the ones really heavy in hay and added fancy stuff like fresh herbs will break down in to pieces.
For now until you figure out the problem and if the teeth do need trimmed, it could also be mouth sores from something or digestive disturbance, you can moisten pellets to make eating easier and encourage more rounded diet with fiber instead of fruit. I absolutely would not feed apple. Anything and I mean anything is better than apple. Fruit juice (watch for too many added sugars or being nearly all apple juice) or pureed bananas works great if the rabbit is being very picky. I've also shaken alfalfa leaves in to the mix and turned it all in to a gooey banana or blueberry pile. Greens would be better though. Dandelions, grasses from a fence line that doesn't get mowed so it's older, daylillies are completely edible from flower to bulb if you know what they are (they run wild most of Iowa), plantains, clovers, safe tree branches... Make sure it's all safe though. Nothing from the roadside and if you question whether your neighbors spray it's best to skip it and buy some stemmy herbs like cilantro and parsley in the store instead. If it is a digestive tract problem the greens will help keep things moving with less contribution to the problem than fruit and especially apples by a lot.
