OP, you're absolutely within your rights to give up your caring role. Caring is HARD and you do not have to do it.
As pp have said, contact Adult Care Services and ask for a social care assessment. Depending on your Nan's needs and wishes, it may be possible for her to have carers visit her at home. If you want to, you could be involved in that to whatever level you feel would be right for you both. Eg, you might want to do two days with her and the rest paid carers or you might want to do none at all.
You don't have to care for your Nan, just because she wants you to. And you can still be there for her, just not doing everything.
It could be that having paid carers come in enables you to go back to a more healthy grandmother/grandaughter relationship with each other, where you see each other for pleasure as well as practical help.
Social services will consider 'putting her in a home' as the absolute last resort because it is so expensive. Therefore, they will prefer to keep her in her own place as long as they can. They may put pressure on you to be part of the caring workforce for your nan, so it would be good to have a clear idea of what you are and are not willing to do now. If you don't want to do care anymore it is OK to say, "I cannot and will not do this anymore. It is affecting life and health in this way..."
Also, as pp said, be prepared to chase and chase. In my experience, it's the squeaky he'll that gets the grease. Social services are so stretched that they can end up prioritising people in crisis (rightly so) but then some people do get forgotten about or fobbed off if they do not have someone to advocate on their behalf. Good luck 