I guess you needed/need a certain amount of progesterone to counteract the oestrogen.
I found this today on a reputable site;
Mirena contains 52 mg of levonorgestrel (LNG). Initially, LNG is released at a dose rate of approximately 20 mcg/day. This rate decreases progressively to half that value after 5 years. Mirena must be removed by the end of the fifth year and can be replaced at the time of removal with a new Mirena if continued contraceptive protection is desired.
So if after 5 years you’re getting 10mcg then after 3 it must have reduced.
I’m in a similar position where I need a bit of progesterone to counteract my oestrogen which although isn’t high is enough to cause me issues. And I’m guessing after 3.5 years the Mirena isn’t giving me enough. X