IUD Replacement. Am I making the right decision?!

Today I had a second visit with my gynecologist, and had an ultrasound done. He informed me that my Mirena IUD has shifted down. He told me because it's not sitting at the top of my uterus that it's ineffective. I looked that up, and because Mirena releases a hormone that's actually not completely true. As long as Mirena is present in the uterus, it's effective. Basically he informed me that my only option is to take it out and have another put in. For various health reasons, I cannot take any other form of birth control and I have a hormonal disorder making it awful to live with no birth control. But the idea of another IUD and it not working is TERRIFYING.