I don't think I'm selfish, I just behave like a normal professional twenty-something without kids yet. I work hard, support myself, spend my free time with my friends or going on holidays as a treat.
That's what matters. What you said here is great. If you have the means, why not go? Enjoy every minute of your trip.

I know it's hard to turn off that inner critic, but you're right--your mother is looking to you to fill the void in her life. Wanting to do things that make you happy doesn't make you selfish. I'm sure that dream has left you nervous (that's very understandable), but what can you do to make sure that it doesn't happen? Is there a safe place where you can put your passport so you know it will be there when you need it, as Grieving_Sister suggested? What can you do to silence that voice in your head telling you that you're selfish?