Part of the problem with fast fashion is that people are no longer learning how to mend their clothes, either at home or in school. So if a hem comes down, a seam gives or a button comes off, it is easier to throw away the garment, and buy a new one.
And the quality of fast fashion is poor, so it doesn't take being mended too often anyway - plus higher quality clothing costs more. So it is easy to get sucked into a vicious circle - buy cheap clothes, then don't last and aren't easily mended, but you can't afford to buy the more expensive, longer lasting replacement, so you get the cheap replacement instead, and the cycle continues. You can mend cheap clothes - I do, and I make cheap clothing last much longer than the manufacturers and retailers would want me to.
Terry Pratchett refers to this in one of his books - the boots hypothesis. Captain Vimes can only afford to buy cheap $10 boots, and these wear out quickly, so need replacing. The hard wearing boots cost $50, but he can't afford that. But the expensive boots would last much longer than five pairs of the cheap ones, so would be a saving in the long run - but a saving you can only make if you can afford the $50 boots upfront.
So, to wean people off fast fashion, we need to make better quality clothing more affordable, and we need to make sure people have the necessary skills to mend their clothes when they need it.