I understand your fears OP, and I'm the same age as you. I've been overlooked for a lot of jobs and I'm convinced my age is a factor. I'm well qualified and quite senior in my field.
Unfortunately, many employers prefer younger employees because they are cheaper are more "mouldable" - and therefore less of a threat to hiring bosses who may well be younger than us. Or at least in my field that appears to be the case.
Not all employers are like this though - my friend got a new job at 51 where she expressed doubts to them about her suitability. They told her: "You have exactly the maturity and experience that we are looking for." She got the job and loved it, but had spent a year looking and had had several rejections. She was very particular about the work she was willing to do though!
I have a friend in her early 60s who does not have a particular profession but she is great at getting jobs and is never out of work. She got a good government job when she was 60 as an admin officer that paid well. She did so well they wanted to promote her but she decided that at her stage of life she didn't want the pressure and left. She's doing care work now looking after disabled adults and absolutely loves it.
Just stick at it, OP, and something will come up. I think the suggestion to do an AAT course is brilliant. If you are reasonably numerate, you've nothing to lose!
I have a friend who left her DH at 69 - she is in her early 80s now. She lives frugally but manages to have an amazing life full of adventure. Somehow you will find the resources you need, as she does.
Finally, why don't you consider a job at Waitrose? I know of a couple of people who work there and they absolutely love it.
Congratulations on making such a courageous move, OP. Fortune favours the brave and it is just a matter of time before you land a role that's just right for you. Have faith. I'm rooting for you!
