Anne,
I have posted on this in the past. My diagnosis was similar to yours, and the advice was ablation and/or pacemaker. I was strongly advised not to supplement with Mg at the levels often recommended here as it has a calcium channel blocking effect, which is problematic with bradycardia (including the brady phase of sick sinus). Subsequently, I stopped all supplements except for 200 mgs Mg, and started to feel better in certain ways although the afib persisted. I then moderated exercise by cutting back on the running component, and moderated eating by reducing intake versus specialized diets, with the exception of cutting out dairy. I have reduced my BMI to the low end of the normal range, and I now have no observable abdominal bloating. My afib has been in complete remission for going on 2 years now.
I would also point out that my afib may have been somewhat atypical, in that although it started with less than monthly sub-30 minute episodes, it progressed to multiple daily episodes of similarly short duration. I never experienced longer episodes. Possibly this variation is less serious and/or more treatable. As for the sick sinus symptoms, I now seem to just have a fairly slow resting HR (45 BPM) which responds normally to increased physical effort.
I make no recommendations and I am just relating my experience.
Best of luck,
Peter