I have a lot of books already, more than I can ever read, so when I buy one it's usually because it improves my selection. When it's time tor pick my next book I have another one to choose from that seems a likely candidate.
Yes I waste money on books I'll probably never get to but it's pretty hard to feel bad about that. If it is a sickness it seems to me to be a good one that I hope I don't get over.
Also as Nicky said, when something I think I'll probably read goes on sale I'm likely to buy it. I've read all the 87th Precinct books, for example, mostly decades ago and I'm sure I'll eventually read them all again. I have several and when another goes on sale I buy it. I'm reading them in order now, about 3 or 4 a year. I'm only 76 so I'm pretty sure I'll have time to get through the next 40 or 50.
Barry