Many straight people too get bit bitter and angry as they grow old, no matter they may have children or not. Getting bitter hasn't got much to do with your sexuality or the fact that do you have children. If "growing old as gay man" causes you distress, I think the real problem is attached to the "gay" part. Maybe there is smth there?
At 50 people usually want to achieve something "permanent". Maybe leave smth behind, or give smth back. If you can't find the meaning for your life, it's easy to crawl back into yourself, get bitter, and feel like you don't matter. When presence feels intolerable, it's easy to fantasize about the past, and give up on future.
When you get old, it's time to look back, and valuate was it a success or not. So basicly, can you accept your past, all of it's hardships and learnings, challenges and rewards. Can you say, that you have live, or will you get bitter because of all of the things you didn't do, and the mistakes you can't repair?
Working with elderly I have noticed, that no matter the financial stature, being a widow, living with out relatives, being sick or healthy, nothing really defines how happy or optimistic the person is. Happiest say, that the real key is attitude towards life. They take part, are interested and don't let minor setbacks put them down. They look forward, and remember past with pride.
Maybe by thinking and worrying, you are going through your values. What really is important to you, what you really want. It might be sad at times, but it's very normal.