"I wouldn't mind offering some support, but I think that everyone is assuming that I'll be 'the one' - the carer".
I appreciate that you've had a great deal of advice. All of it sound, sensible, and with no dissent! Hope you won't mind my commenting late! That sentence in your opening post made me think immediately "who's everyone?". Sadly, as your later posts explain in great detail, his family and presumably he have already made this assumption about you. You, your needs, your wishes, your plans don't feature because yes, you're lined up as the carer. So, leave it much longer and your fate could be sealed!
He has been in your home for over 2 years - this was to allow some refurbishment of another home to be completed. Then there was lockdown. OK, but now - in my view - he's really taking liberties. And since he is paying you a regular sum each month he possibly feels that he he can go on living under your roof, guilt-free, whilst the home in which he is destined to live is now a rented out, nice little earner for one of his children.
I'm sure your Counsellor will be of great help to you, but I don't see this decision as difficult or complicated. You may feel it's difficult because you're clearly a very decent and caring person. He doesn't even want to think about it!
With or without big girl pants, you now need to understand that you ARE being taken advantage of, you DO need to just tell him that when you allowed him to move into your home because of some inconvenient circumstances, it was to help him until the house was straight again. You now want to return to living on your own and he can sort out his future from his own property which is not only finished but has a tenant !
There's nothing unkind in that, you are being perfectly reasonable. You mustn't allow him to become a 'victim' and he needs to return to facing life as a widower, running his own home and own life. That's not your job. The 'assumptions' need to stop - and you probably know this. Don't put it off and please don't even attempt to discuss his future plans, or any concerns you might have, with his children or anyone who knows him. That simply makes it look as if you feel you have some responsibilities towards him. You don't. They can assist him or not, their choice, their own family business.
If you decide to continue your relationship with this man on a different basis now, or if you feel that it has run its course, you'll be able to make much better judgements without feeling resentful that your life with him has become a job, a responsibility, that at your age you really don't need. Continue to heal from your sadness and loss, make a better life for yourself where you have freedom and the happiness you deserve. And please don't agonise over a sensible decision which is long overdue and which is really going to make you feel a whole lot better. Good luck with that new life!