Her dementia is getting worse. I am an only child, and live 70 miles from my mother. I try to visit at least twice sometimes three times a month and call her twice a day. I try to divert the conversation after I answer her but she always sounds either down or mad.
By all means tell her the next planned date, and give her a big, bright calendar with visits marked on it; but more important than that when you speak to her sound enthusiastic and tell her you can't wait. Talk about the visit, and what you'll be doing together, not about the reasons why it can't be sooner. Say you'll be bringing her a new photo/book/ice cream/hand lotion, and how much you're looking forward to it.
Um. I'm not sure how much good the twice-daily phone calls will be doing her - they may cheer her up but they may not, and if they're getting you down this might be something you can cut back on without hurting her. Ask the staff who know her best whether your calls help to structure her day, or whether they make no obvious difference. For example, it could be that she asks for you all day long, hour after hour, whether or not you've just called or visited.
Are you happy with how well she's being looked after generally?
Don't give her a specific date when you are coming. Maybe just showing up is better. I assume she can't probably handle really understanding when you're coming and/or how to track it on a calendar, etc.
Maybe you should come up with a canned response to her inevitable question. Something nice and vague and quickly change the subject. Have it all planned and use the same answer every time. Or change it up to entertain yourself.
You can also just tell her when you get some free time, super busy and can't really plan because I just don't know. Encourage her to get or stay active and engaged in the facility life. I'll see you as soon as I can mom.
Don't tell her you are retiring or when you do. She will think you can pop in everyday without a job.