I understand the fear Iasa, I don’t know how great your GP is but it is best to tell them your fears and expectations (e.g. only females attending) and you may need to repeat that to booking and to whoever is fitting it on the day. Unfortunately most gynae type places will have treated assault survivors before, they will be sensitive to your feelings.
PP have given the pros/cons of the mirena itself and suggested practical things like diazepam or painkillers, so from an emotional angle:
Prepare yourself by remembering that it is a medical procedure and not the same situation as the assault(s). The doctor and nurse are coming from a place of care for you.
Having a small ‘mantra’ can help during the procedure to repeat to yourself, or a stress ball (since handholding will be out) and the assisting nurse may also try to chat with you to distract you, and importantly, if you say ‘Stop’ they will. You have choices, you are in control of your own body.
Clear the rest of your day if you can, have easy, tasty food in the house and maybe hot water bottle, blanket, film, because you may feel upset/tearful after its all over and you’ve gotten through it.
Or you may feel uplifted as its one more ‘first’ after the attack(s) survived and dealt with (first time leaving house, first day back at work, first conversation with a male etc) and one more step in your life that leaves the attacker further behind you and you further forward in your recovery and life.
I hope it goes well for you Iasa it isn’t easy, but it is probably worth it for your physical & mental health once its done.