Why is this bothering me so much?

Just for reference, I am a male in my early 30's. Since the summer I have started to work on several issues from my childhood that I had been ignoring for a long time. Part of this process has been talking to my mother, who was my primary caregiver as a child.

One of my issues is that she was incredibly cheap to a ridiculous extent and this affected me in several different ways. During a recent discussion, she essentially challenged me to give her an example. I brought up how despite the family having tens of thousands of dollars in the bank she made me wear my older brother's used underpants as she did not want to spend the $2-$3 it would have cost to buy me new ones. I even brought up a memory of telling her that I didn't want to wear them and her telling me back that as long as they were not stained I didn't have a choice in the matter.

Despite this being an incredibly light incident compared to other things that have come up, It has still been bothering me for the past few days and I am not sure why.

How did she respond to the underwear example? Is she combative on every point? Does your sibling(s) feel the same way? What are you hoping to achieve (to have her acknowledge it)?

Anyway, I grew up under similar circumstances, though not completely the same - and I grew up with my father.

All my childhood passed under similar conditions. And it causes certain psychological problems even now.

The key is to acknowledge why this makes you feel bad. And to understand her reasons (even if they are strange) for doing it.
You can see, it was not good for you psychologically. But it seems true to me, if a mother is doing things like that, probably she is a bit 'abnormal' too.

It is a pain, I think most bad parenting, the parent does not even understand why it was harmful. They often did not have awareness of the implications of their behaviour. That is not an excuse, but it maybe can help to see: "Well, she just was very confused."

It could be very difficult to get a mutual understanding with her about it. It is not ideal, but you may need to only reach your own personal resolution about this, regardless of how she feels or responds. Although if it does not negatively upset you, it is brave and good to keep trying to explain to her how you feel about it.

I would say that it seems as though you already have gained a lot of understanding. Maybe you are at a point where this is complex, but it seems like you will work it out yourself, to my reading.