I'm not quite as old you but close, and I already feel what you're feeling about the time left. That there isn't a lot of it, or that it will be gone quickly. Everyone has things they will not get a chance to try or experience. No lifetime offers everything, and every path taken means many, many others will be never explored.
Like money, you can't take memories with you. So try not to dwell on things you didn't do or that didn't work out the way you imagined. Half or more of people who get married end up divorced. Probably many more are less than happy. Kids can be a joy but they can also be a heartache. Every criminal is somebody's kid. Nothing comes with any guarantees.
Make life interesting today, as today is the only thing you really experience.
If I may provide a suggestion... search for a serious website that is about dating (not tinder or something the like) in your area and connect to people and try to date (meet in person for at least an hour or so) at least one person once a week. Don't spend too much effort in people who just want to stick to online conversations or fooling around, just move on. If at the meetup there's no click, you have a good evening or at least you built up experience in dating.
You will need to get probably seriously out of your comfort zone but it becomes easier each time! Practice makes perfect!
But that said, there’s alot of philosophy and other things to help. Aeschylus said “Happiness is a choice that requires effort at times.” Another relevant quote is that the best time to plant a tree was 20 years ago, the next best is today.
You won’t get what you wanted, but live in the present and enjoy what you can have. I’d give anything to get my wife back, but that’s not reality. The next best thing to live today and find joy.
Could you elaborate on that? As someone who is younger I have noticed that many (but by no means all) people over 60 often do not want to commit to these kind of 'life-changing' escapades, despite now having the time (kids out of the house and/or retired). I assumed it was more around lack of (youthful) energy/health and the fact that you're so used to how you've lived for decades that change is far more difficult or feels more daunting.
You're saying that having a relatively shorter amount of 'time left' makes such a move different, wouldn't that fact make it easier? (YOLO and so on...)
Moving to a foreign country with an unfamiliar culture and language, and establishing a new life there, is a chore. Probably even more so if you don't have a spouse or something who's familiar there:
Learn the language, the culture, establish or reestablish your daily habits, find new friends and ways to socialise, perhaps new hobbies if the old ones aren't available. Find out where and what to shop and how to cook, when the stuff you know is gone. Turn your house into a comfortable home, etc.
If you're older you might not feel that you'll get to see much of the payoff.
I also imagine it gets harder to find people who are open to new friendships, as you get older.
If it helps: https://www.7cups.com/
I'm truly sorry about all the bad memories. I do know what that is like, many what-ifs. If you're receptive to any thoughts: Mourn them, but try not to waste the present being angry about the past, or you'll regret missing out on this time too. Give yourself another shot, try again. There will be a time where you don't get any more chances, but it's not yet. I know post-70 year olds that have cycled up mountains in France. One that just stopped being a climber after two replaced hips. I'm sorry about the kids. Maybe travel and meeting someone is still an option. Take care.
As for the topic itself. The obvious reason time passes faster when we're older to me is that each day is less of my life. At 1 week old a day is 1/7th of my entire existence. At 60 one day would be 1/22000th of my life.
I also feel it in terms of time left. When you have $1000 in your wallet, splurging on a $50 meal might seem fine. When you've got $75 in your wallet you're unlikely to blow $50 of it on a meal (unless you've got a supply elsewhere). In the same way, when I've have got 20yrs left in my life, some of them probably not in the best of health, then committing 10 of them to move to a foreign country to immerse myself in a new language feels very different than when I've got 60 years left (20yrs old). Seeing your life left clock go down 1/60th (1yr at 20yrs old) feels slower than seeing it go down 1/20th (1yr at 60yrs old). that's 3x faster.