Trouble mounting my tb x mare?? HELP?

HI, as title says i have a tbx cleveland mare she's 14yrs old ive had her for 5 wks now but sits been a nightmere, we've moved yards already as the old yard had rubbish hacking and a tiny school. Now ive ridden around 5 times since i got her frist time was fab all rest including today have been hard work, she is scared of mounting block and even with someone else holding her just moves when u trys to mount her?? looking for any ideas or surgestions as to how to help this problem