It's very commendable that you are willing to help your friend out like this. The best way to get some relief from this situation would be to cut back your visit to every other week or even once a month. That would give you time to recharge your batteries between visits.
On the other hand, your friend still needs emotional support and practical help. Try to enlist other friends or relatives to share the load. Let your friend know that you still care for her very much, but that it's hard for you to be there for her every week. Ask her for some suggestions about friends or relatives who may be able to help out.
If she can't think of anyone, there are community resources that can be enlisted. Try your local social services department for different things like Meals on Wheels. The local hospital may be a good source for finding volunteers who can help. And if she has a place of worship, certainly her congregation can be contacted for volunteers to help out. Many times it's just a matter of speaking up to let a person's needs be known.
She is very privileged to have a dear friend like you, but in order for you to be there for her, you have to take care of yourself too. Let me know if you need to discuss this further, I would love to hear from you.
If my answer was helpful, please click the Accept button. If not, please click Reply.