Hi!.
Yes I remember your problem well.
SVT that is not responding to conventional treatment and to cut a long story short, you have had an ablation which has not been successful which is unusual but it happens.
It is not my area of expertise, but I thought that an ablation and "zapping "of the A-V node was the same thing.
Whichever.....you seem to have come to the end of the road and a pacemaker is the likely answer and are easy to insert, last around 5 years in duration and are "life saving" in many occasions and in you condition would not hesitate to have one inserted.
" What happens though if pacemaker fails?"
See the glass as half full and not half empty......pacemakers have come a long way and the outlook is excellent.