Are you on estrogen? There is no other reason to have a mirena IUS at 65, and if the palpitations began when this was inserted I would consider removal - a device passing through the cervix could be irritating your vagus nerve - this is also what causes some people to have a vasovagal episode on insertion or removal of a coil.
If you are on estrogen you could consider a non-invasive progestogen instead.
Hypoestrogenism and thyroid disorders can also trigger palpitations so I would rule these out first if you haven't already, as well as checking electrolytes.
Magnesium and potassium supplementation can also be helpful.
If a cause can't be found there are other cardiology medications apart from beta blockers which you could ask for.
Taking sleep medication for insomnia that is caused by palpitations rather than addressing the cause would be a last resort, however some people do find melatonin helpful.