I think there are a few things going on here. One is that the horse is still on the "youngish" side (as I am learning with my 5yrold). And, I missed just how much training she has had in the past. Two is that you seem to be still learning about how to handle / discipline. I might be way off there, but I got the impression that dealing with this type of stuff is new to you. Without going back and reading the thread again about your latest work with Chance, I think it would help you to have someone else work with Chance for you to see how they handle the same issues that you are recognizing. And, you don't need to think you are not big enough to handle her unless she really towers over you. This is not about strength. It's about repetition, reward and, when needed, leverage. If it's about your size vs. her size, you have already lost. Your advantages are your brain, your memory, your perserverance, your consistency.
I remember that you said have spent several months with Chance. How has her behavior changed in that time? Better? Worse? And don't forget there are always those hiccoughs in time where even though last month was great, this week the horse might be a nightmare!
I don't think that riding her is too hard on her unless there is some pain that you are not aware of. However, that said, if you are more comfortable doing ground work with her, I would encourage lots of it. If you focus on some of the main issues first, the smaller things will fall into place more quickly. If I remember correctly, a big thing was basically respect for YOU (your space, your presence). Spend lots of time reinforcing respect and moving away from pressure. That means when you ask her to walk forward by forward pressure on the lead, as soon as she moves in the right direction, release the pressure. Use the pressure and release method LOTS and LOTS. Also, it might help to teach her to lower her head on command -- again pressure and release. That's a tough one, but a good one. For the grazing, develop a command that tells her when it's OK for her to munch while on the lead or with a bridle on. For my horses, it's usually, "OK, go ahead" (stupid, but they know it now). Until I say that, they are expected to stand beside me quietly. If the head goes down too low, they feel pressure on the lead. I don't exactly pull on it, but there is resistance that only goes away when the head comes back to an acceptable level. I will pull if necessary, but the reward is in the release.
Sorry... I think I'm rambling. Basically, I think you need some guidance and no matter how much we talk on the forum, there is nothing quite as good as learning from someone else that you trust working with your horse and showing you what will work for her and what you can do.
Keep your goals simple initially. You may indeed trying to be doing to much if you are doing lessons on her for yourself and for her at the same time. You weren't clear on who the lessons were for. I would get her respect first and don't ask for anything where you think there might be trouble. Take it slow. If riding, ya, keep it at a walk until BOTH of you are ready for more.