So, no matter how hard you may explain to her about her present condition, that will not help .
Secondly, applying force to get her see a psychiatrist may seem to be an option, but mind t your daughter is an adult and the whole idea may backfire as this involves some legal hassles as she may accuse you people for undermining her rights and believe with her underlying psychosis it will not be odd if she does that.
Also, since she has delusion of an affair with a high profile celebrity, so application of force will only draw much curiosity and attention from the onlookers and will not only make the whole issue a talking point among the neighborhood , but also will make the very personal and family issue ,an issue of public interest and this may also put label on your daughter.
So, finally we are left with one option and that is you consult a Psychiatry Hospital or a Nursing Home or for that matter a Psychiatry helpline service, these agencies deal with such kind of issues on a very common basis and they know how to deal with them, as they may send out their health workers to get your daughter admitted , also this will evade any kind of legal hassle. Also, you may seek help from a lawyer on this very website as to how you can take decision on behalf of your daughter , since she is psychotic and lacks insight.
I hope my answer serves your query according to your satisfaction.
There is a green accept button that you have to press , so that i can get my due.