I can almost identify with you. I have lost both my parents, no siblings, and have no children. I do have a partner, and I know that without him I would be utterly lost. After my previous relationship broke up, my ex managed to alienate me from all my friends, (had already moved from the north of england to london when I met him, so all my friends were in the area he lived in). Have now moved again with DP for a fresh start in the South West and dont really know anyone, but I did get in touch with my only cousin, who I had lost contact with over 20 years ago and we are rebuilding our relationship. She is married with children though, and they are their own family unit.
I am 46 now and finally accepting that I wont have children of my own, but fostering is something that I may look into in the next few years as we have the space, time and love to give a child.
Do you like animals? A dog is a great social tool for getting out meeting people.
There is also nothing to say you wont find another partner. 25 years was a long time with someone and you must miss him very much. You are certainly not too old to meet someone else though. My mum was in her mid 60's when she lost my dad and she continued to go ballroom and latin dancing with her friends, and met someone who she then spent almost 15 years with until he sadly died. But in that time I know they had many happy years and a lot of fun together.
Life is what you make of it, and the only caution i can suggest is not to seem too needy to any potential partners when you first meet them. (I have a friend who blows every new relationship in this way as she goes in for the kill as soon as she has been on a couple of dates), and equally be wary of any man who comes across as saying you are the one and hes thinking marriage etc within the first few dates too. Take things slowly.. and good luck. :)