Sure, a loner can get to Heaven. Look at all the hermits out there who became saints.
I think there’s some mistaken perception out there that if you “only pray” that’s not enough and you should be out joining 10 groups and volunteering at the parish. I disagree with that view. Prayer is very important, sounds like you’re doing that. And you’re also tending to your family responsibilities by babysitting.
“Thinking about death a lot” can be okay if you’re just coming to grips with the idea of dying or with having lost loved ones. Or it can be not okay if you’re thinking about how much you’d like to die or becoming depressed or morbid. Maybe think about whether pondering this thought is healthy for you, or if there’s some other interest you can pursue on your own, like reading about a subject you really like, or self-study, or crafts etc.
I’m alone much/ most of the time too. I have other friends on social media who live at a distance from me and are kind of in the same boat for one reason or another - their families have died, they’re at a distance from or estranged from family they do have, they didn’t happen to get married, etc. Lots of loners out there. God likes them just as much as he likes extroverts.