Smileless So sorry to hear about Mr Ss Mum. If she is sedated then it should be a gentle time of passing.
Nicegranny It is good to take responsibility but to do that you have to know what you are taking responsibility for. I am sure that what you describe will have impacted on both your children to varying degrees and will have been hard for them and for you.
But IF the difficulties in your relationship have arisen since the arrival of his fiancee then maybe you need to look at what, if anything, you have done that might have caused the problem for her and for your son that you can take responsibility for. Think about any behaviours, incidents, comments from THEIR perspective. It might help you to either:
a) see and accept at least partly where they are coming from
OR
b) accept that this is not your responsibility but his choice within his circumstances in his relationship
If you can draw any conclusions from the above considerations, then it might help you to move forward, still painfully but still forward, rather than circular. 
Only 50% of middle age adult manage more than 1 brisk 10 min walk a month.


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. He had clearly been spooked by something as although some got to within touching distance, he kept running off.
. TBH your friends behaviour has left a rather unpleasant taste in my mouth.