I've been shunned by most everyone because there's something wrong with me and no one wants to tell me what it is, even therapists I went to.

If it's a total/near-total shunning then it's almost certainly your looks. That level of shun is reserved for unacceptable/intolerable or disgusting visual differences.

I understand that there are good aspects to older adulthood, but how will I even be happy with that when I look back and see that the years that were supposed to be fun are filled with pain and isolation.

The lame old 'get busy living or fucking die' cliche can be applied here. You're still young enough to where maybe you can fix things, but after you pass 30 you're pretty much boned, so either start fucking shit up or fuck off, IMO. I did that myself 10yrs ago at your age. It didn't really work out and I ready to leave, but that I tried feels/felt better than being the fearful, timid, doormat shit I was before that.

If you're going to snuff it, then just fucking do it; if not, then you have go out in the world and fuck shit up, have to try to hammer your life into what you want it to be. If could work, who knows?

^{Shortly after my 25th birthday I was soo close to taking a pill cocktail, just waiting on the final ingredient, and decided that since I was just going to kill myself anyway, why not just do what-the-fuck-ever and stop being so fearful, wasting my life, and hating myself. It worked so well that I gave myself a year to try it out, telling myself that if it didn't work I could always choke down the cocktail and go out with nap. Turned out that life became enjoyable for the first time and I breezed past that 1-year mark. Anyway, here it is 10yrs later and it's all turned to shit, but I did wreck shop and tried every damn thing I could think of to make life okay.}

Jus' sayin', who fucking knows.

become a "mature adult"

Unless you have kids or dependents to manage, then you don't have to do that. That's one of the bonuses of being childfree.