I think a big part of the problem is that clothes simply don't last as long as they used to because they're not as well made. If you're on a limited budget, it's very easy to get sucked into a spiral of buying shit that constantly needs replacing, and no amount of preaching about making things last is going to fix that.
Also the op has already says she uses Vinted, Ebay and buys in the sales.
I agree about getting things altered and repaired, especially if you struggle to find things that fit. If you can learn to do your own then even better, although it takes time, which is a cost in itself.
I try and buy from real shops as much as possible. I know I know, it's hard, given the state of local high streets, but I think it's much easier for online retailers to fob us off with badly fitting crap than it is for a real store where you can look at and handle the garments. The same applies to second hand clothes - I buy them a lot but only if I can see, feel and try on first.
I think it's worth working out which items are worth spending a bit more on and which are not. It's an individual decision that depends on you and your lifestyle e.g. some people might advocate investing in shoes but I don't because (with the exception of Solovar) I trash them really quickly. (But I do get shoes repaired regularly which extends their life.)