I am with all those who say stop paying for holidays and any other luxuries you are currently paying for.
You say your daughter is supporting herself, her children and her partner, so I don't understand why you are worrying about her mortgage - is she not able to manage paying that?
If you have LENT your daughter and her partner money, either jointly or separately it is not interfering to ask when you can expect them to start paying back some of it. If you have GIVEN them money or they reasonably could have felt that what you call a loan was a gift, then you are facing a more difficult conversation with them.
I would sit my daughter down and tell her I no longer am in a position to pay for holidays etc. and offer to go through her finances with her, or go with her to her bank or building society if she needs to discuss matters with them. In no circumstances should you agree to stand guarantor for a loan or a new mortgage.
Try not to sound critical of her way of managing or of her partner, but it should be possible to ask nicely whether her partner's business is in financial difficulties. Tell her that you are willing to do what you can to help (except lend any more money, or pay expenses) but that you do not feel you can help very well, as you do not understand her precise situation.
I am very much afraid your daughter's man is a con man, but if he is, she will need to realize that for herself, as anything you say on that subject will offend, as it will if you ask whether he gambles, drinks, is paying child support to children you have never been told about, or any of the other possibilities that are keeping you awake at night.
I hope you manage to have a constructive and friendly talk with your daughter. Do ask her next time she moans, what she is going to do about the situation? If she says what can I do? or I don't know then ask for details so that you can advise her.