3 years. I had other side effects that I never attributed to the IUD, but nothing that was serious enough that I would have removed it. They had been getting increasingly worse over the last year or so that I'd had it--but I really only realized that afterwards, too. I had been increasingly anxious looking back, too, but rarely to a point that I even thought about it.
One day, FOR NO REASON, the anxiety hit me like a ton of bricks and didn't let up until 7 weeks later when I had Mirena removed. It was 24 hours a day and medication didn't really help. Anti-depressants made me worse (I'm apparently over-sensitive to serotonin-based meds). I spent 5 days in a mental hospital. I was a mess.
Keep trying different things...something will work. I felt like my body got used to being anxious and I had to break the cycle. Does that make any sense?