Someone asked me this question on Quora. I thought it might be useful to share my reply here as well so others might be helped with the information.
How difficult it is depends in large part on you, the care partner.
If a care partner is devastated and grieving over the fact the person they love who is living with dementia doesn’t recognize them, I think she or he will find it much more difficult and stressful to provide good, loving, compassionate care.
On the other hand, if the care partner can let go of his or her ego, and understand that being recognized by name or by face is really not that important, then I believe he or she will have a much easier time as a care partner. She or he will be less stressed, more relaxed, more loving and compassionate, more open to going with the flow, and more able to find joy, happiness and fun.
These two links expand on the idea: