I'm not bragging. You should be able to tell from what I've written that I'm not proud of what I've done in life so far.
Is it wrong for me to prioritize getting some real stuff done over having a car, house, or girlfriend? It's not like a gas-guzzling vehicle will make me any happier, and I find gardening distinctly unenjoyable. No comment yet on this relationship stuff.
Regarding the prioritization of cars, houses, and relationships....
I would say that owning a car is just what it appears. If it doesn't seem appealing, don't do it. Owning real estate, pretty similar. Moving out, maybe a little more subtle (i.e. the costs/benefits are less obvious to someone who hasn't done it yet), but not enormously more subtle.
But I would argue that girlfriend/relationship is not like these things. (Obviously I speak here for my own experience, and I claim that it's somewhat generalizable.) Relationships are rewarding in weird and surprising ways, and difficult in weird and confusing ways. Also, they require practice to maximize your benefits, so if you figure you're going to want this at some point, getting some low-pressure practice in might be prudent.
Something that makes a difference in the lives of real people. In all honesty I don't know any more than that, but I have a hard time believing that it's any of those things you mentioned.
The best way to see what makes a difference in the lives of people is to get out and meet them.
A good start would be just gaining some independence for yourself. The world suddenly looks very different when you remove most of the safety nets and have to sink or swim wholly of your own resources.
I personally don't think you should be considering world-changing events right now. You should be looking towards simple things and expanding your own horizons. You can split the difference by volunteering at some local organizations and maybe joining some groups/clubs that seem at least mildly intriguing.
BTW, in my prior post I didn't mean to imply that I thought you were bragging.
I think you mostly have existed and not actually lived. You should probably start by just gaining some independence and some outside extracurricular activities.
You should have a house/apartment, a car, a girlfriend, some hobbies. Lack of these things and the ability to live for 10 years on any amount you could have reasonably amassed as a 24 year old corporate programmer is most likely not a topic to brag on. This should be an indication that you need to expand your horizons.