Your mother has presumably had a couple of decades, or more, more than you to acquire the habits she has. Habits become more difficult to change the deeper they are ingrained. You show a willingness (eg this very post) to adapt that we have no evidence is matched by your mother, so I think that has to be exploited. That is, it is rather up to you to 'comply' (and credit to you for showing some willingness to do so).
I think @Willeke's advice is a large part of a solution but may still be a little more confrontational than necessary. I am assuming your mother is less up-to-date than you are with the current contingency facilities available to travellers. For example, shops that open 24 hours rather than closing half day Wednesday and all day Sunday. And overnight delivery from online stores rather than several days by post, and back up that a mobile 'phone provides more effectively than fixed lines may do while in transit. And the ability to purchase with plastic even when cash has run out or been stolen.
I doubt your mother is bothered by any extravagance from buying an extra pair of socks while you are away (because yours got wet and you only took one pair). I think what she is concerned about is your discomfort and health from spending too long with wet feet just because you do not have a replacement pair of socks to hand.
I know (personal experience) the improvement in relationships achieved by "fleeing the nest" for a short period. Prove you can survive on your own and your mother will be reassured that she no longer needs to involve herself in your well-being to the extent she has for over 20 years. Just one trip away as an independent (with minimal luggage) should be sufficient evidence.
However for that first solo trip you still face the existing issues. I suggest you pack what your mother wants you to (avoids confrontation) and then take steps as suggested by @Willike. 'Accidentally' leaving a bag of surplus material behind may not be a good idea however. There is bound to be suspicion the forgetfulness was deliberate (exacerbating the issue) and I would expect your mother, from what you say, to make very certain it was not overlooked.
Better would be compare, on your return, a pile of what you did require with a pile of what you did not. And maybe mention the inconvenience you suffered from too much luggage (eg cost of storage). If not practical to split into two batches and leave one with your mother while you are away (despite her protestations!) try depositing what you do not want to lug around at a left-luggage locker. Then show her the bill for that, or a picture of the receipt, or of the left luggage office.
Also, you could deliberately leave behind, before setting off solo, one or two items you do expect to need. Then on your return recount how you managed without them. Perhaps: "I used a comb instead of a hairbrush", "I washed my hair with soap instead of shampoo", "I 'borrowed' some loo paper and used that instead of a handkerchief", "The train station sold umbrellas". This won't stop your mother worrying but should prove that "push comes to shove" and you'll manage. Assure her of that and I guarantee fewer arguments about packing thereafter.