OP, unfortunately the course sellers make their income off course selling, they often don't care if their course is effective or not, since they can easily fake testimonials (or at least cherrypick the few good ones.)

I'd recommend a regular part-time job you like (it can be online, but in-person is better for socializing) and then trying out some side hustles. T-shirts seem like a tough sell and oversaturated, you might be better off pivoting to something else. I'd say the best tactic is joining communities, like /r/passiveincome, /r/workonline, etc.