Happiness is a state of mind, it has to come from within you, and not be dependent on your situation. That is obviously very hard to achieve when you are not where you really want to be- married with a family. R’ Ezriel Tauber once gave a talk on this issue- I am summarizing what he said, the way I understood it and the way it helped me when I was still single- everyone davens for their yeshua- whether it is to get married or have children, and sometimes the answer is “not yet”. Until that time comes, instead of thinking of your current life as just a waiting time until your tefillos are answered, you have to realize that you are exactly where and who Hashem wants you to be at that moment. That doesn’t mean that you give up on the dream and the tefillos, it just means that you are ok as you are, for the meantime. You have to find your tafkid/fulfillment now.
And by the way, marriage does not always mean happily ever after. It’s not a happy ending, but a new beginning and it can bring its own sets of challenges and issues. Unfortunately, older singles who married late can still have other problems/challenges as a married, just like everyone else. But once you figure out how to be happy despite what life throws you, you can face those challenges too. maybe that is an advantage of being an older single- you learn how to face challenges and overcome them.