I hear you OP. I'm the same. Long time single (11 years since I got out my last relationship). No family. Divorced, no kids. Retired so no job (am early fifties). Like you I have a dog.
I think I am naturally introverted so probably cope better than most. My dog is also my lifeline and forces me to go out 2-3 times a day which makes me appreciate getting back home if nothing else. Also he's a funny wee chap and often gives me small moments of happiness by being just a general furry nutcase. However he is 12 this year and whilst he will likely have a few more years hopefully (he's small and crossbreed) the happiness now is tinged with fear of him dying.
I will probably have to get another dog cos quite honestly he does give me a sense of purpose and makes me feel needed.
It is very tough though.
My last relationship just somehow broke me and I just don't think I can go there again. Weird cos no doubt he would say it was all my fault but that does not stop me feeling the way I do.
I was involved in looking after a family member for the previous 5 years and it was a huge strain mentally and physically not to mention grief. I put on a huge amount of weight which I am working towards getting my health in a better place.
Added to that just a few other things that have really made me sad like a good friend who got married after being long time single and dropped her friends which was quite devastating as I thought we were close. I tried to keep in touch with her but it was better to accept the friendship was over than being made to feel crap when she couldn't fit me in for 12 weeks and then only cos her other half had a day out with his friends or something.
Also a family member who I helped through their divorce etc, promptly got themselves back on their feet and then made it clear I was no longer needed.
All these things whilst not personal (people are just looking out for themselves) do make you feel why put yourself out there.
It is very tough in the holidays when you see others with family arriving to visit etc and you are on your own (although I don't count myself as alone with the dog as he makes me feel so much better).
I do have a few nice neighbours who will have a wee chat when we meet and sometimes other dog walkers will speak although I find since covid it happens less. I think cos there are so many dogs now people don't bother anymore whereas before having a dog almost meant other dog owners sort of saw you as a friend to stop and chat to.
Anyway no good moaning as the only person who can fix it is me but for those out there who were born into stable, loving supportive families and who grew up and keep in contact regular with them just know you are very lucky.
Both my parents had mental health problems for sure and resulted in a very dysfunctional family system and impacted all our health as well.
Hugs to you OP.