Warfairn & INR levels

Hi, I've been taking warfarin on & of for past couple year. When I was 18yr I was told I had congential heart disease, I'm now 28yrs & have had open heart surgery 4 times. What started as a hole in the heart, lead to a dual packmaker, had cardiac ablastion, & had two heart valves leaking, at one point was heading for cardiac failure. They repaired one of the valves, the needs to be replaced. How ever I'm currently in atrial fib & awaiting cardioversion, as anyone had this? Whats it like? Does it work? Also I've been taking warfarin for 9 weeks & the INR was stable ranging from 2.63 to 3.21, all of a sudden dropped to 1.55, I don't know why I haven't done anything different, is there anyone who could advice me, & take some of this worry of my shoulders, i'd be so grateful, my anticoagulant nurse doesn't offer much support. Tks