Hi
I spent my 40th birthday in Australia with JY, my 41st last year with JY in Zambia. I started doing these trips in my early 30's in the same situation as you - my travelling companion got engaged and my other friends were either settled in a relationship or unable to afford these holidays. I've even continued to go on these holidays when I've been in a relationship (of course leaving him at home!).
It is normal to be nervous about taking such a big step but if you have a look through other posts you will see that the nerves soon disappear once you start your holiday. The company really looks after you - everything is organised for you, nothing is too much trouble for the Tour Managers.
The best way to start is to think of where you have always wanted to go. You can help settle your pre-holiday nerves by thinking if all else fails at least you get to go to a dream location. Once you've decided on a location don't think about it -just book it. If you're lucky you will find someone on the forum going on the same holiday and you can get chatting beforehand.
Be brave and take the plunge - I promise you won't regret it and will have the time of your life.
Sally