Secondly, stop trying to define your life by "achieving". I know this is a difficult one, I'm still (sort of) struggling with this one. Achievements are ephemeral. Only an incredibly tiny minority of people achieve anything really significant, and even then those significant achievements often only touch a small minority of the world. It helps to bear in mind that about 99.9999% of the worlds population have no awareness of your existence.
The problem with trying to achieve something awesome is that either you never manage it; in which case you are left permanently unfulfilled and with a sense of failure; or you do achieve it, in which case: what then? You are suddenly left having to find another goal.
I think it is better to just find small ways to enjoy your life and work as much as you need to support those interests.
I am 51 and I still go to see bands at pubs regularly. I know people in their 30s right through to their 70s who are still going out watching rock bands. Hell, I know a couple guys who play in bands and their mum, who has just turned 90, still goes out to see them play and likes a dance. I also go to little festivals with friends who are all in their 50's and 60's.
If you are feeling too old when you go to bars and stuff you are probably going to venues that have a younger crowd. Look around and you can find plenty of places that are patronised by people that are a similar age to you. Many of those, both male and female will be single and probably feeling some of the same things you do.
Finally, don't feel alone in this. There are vast numbers of people who feel similar to you. I am single and have been for a very long time, but just recently - completely by chance - have sort of stumbled into something promising. I also handed in my notice at work recently and finish at the end of the month. I am taking a year off to just enjoy myself and am very excited about it.
Good luck, and don't beat yourself up about stuff. Just remember, we're all a bit lost and stumbling through life the best we can :-)