Your mom does sound an awful lot like my mom. I don't think she--my mom--even realizes that she is being selfish. If you asked her, she would describe herself as a very giving person I'm sure. And indeed, she would give me the shirt off her back if I asked...and then remind me of it any time I dared say "no" to her in the future.
In my mom's case, I know it is a self-defense mechanism. She has spent the majority of her life getting taken advantage of, and now she is determined that it doesn't happen to her again, so she is very defensive. Added to that, the last few years have been very hard on her, which has made her even more touchy and defensive.
I think she spends money in order to try to make herself feel better. That woman can go into the Dollar Store and drop $40 without even thinking about it. She has credit cards out the wazoo. One time I was with her and we stopped at Home Depot so she could make a credit card payment. I waited in the car. She came out of the store with a $15 cookbook on cheesecakes-an impulse buy. I think she bought it because she was in a depressed mood and it made her feel better. We got home and the cookbook disappeared in the clutter. She's never used it.
I think the blaming everything on someone else bit is a self-defense mechanism too. Again, she was told most of her life that she wasn't important, yadda yadda (my stepdad was a real winner), so now she doesn't want to ever be the one at fault. She recently failed a lifting test for a job she applied for. She told me that the tester wrote down that she only lifted 35 pounds from the floor when she really lifted 40. Do I believe the tester lied against her? Of course not. She has similar stories for other things that didn't work out in her favor, but of course aren't her fault.
Through much of my life, I have felt like I was the parent and she was the child. She is very dramatic in general, and pouty when she doesn't get her way. I kind of wonder if that's what worked for her when she was young (whether with her parents or with boyfriends) and so she still tries to use it today...but her parents are gone, and youth and beauty are not on her side any more, lol.
I don't always have answers for how to deal with her. I pray about it a lot, lol (and I've seen some great answers!). My husband has had more of a backbone with her than I have. They "had it out" one week when she was staying with us and househunting (she recently moved to OK from Florida where I grew up). It was about something stupid, like what tv show to watch, or something like that. But it was the catalyst to deal with clashing attitudes and personalities. When it first happened, my mom went into her room and started packing her things. She then turned it on me, saying I shouldn't let my husband talk to my mother that way. I just stepped right back out of it and said the two of them needed to work it out because I wasn't going to be caught in the middle of their arguing for the rest of my life. So they did talk/yell. It ended tensely that night, but things got better afterward, and now my mom is much more considerate than she used to be toward both me and my husband. And I'm not so worried about developing an ulcer!
Since then I have found the nerve to speak up to my mom about a few things, like how she always runs the conversation and it's hard to get a word in. She denied it and said she doesn't talk too much...but since then it seems to me that she has made more of an effort to ask me about how I'm doing and not make the convo always about her, and the conversations are much more two-way than they used to be.
So I guess I said all that to say, you may just have to have it out with her at some point. Maybe she is not aware how she comes off. Maybe she's forgotten what it's like to drag small children to a shopping center. Maybe she just needs a wake-up call.
One more thing--does she live alone? Sometimes people who spend a lot of time by themselves don't realize that they've changed and become more self-centered, because most of the time there's no one around to include, so they really are self-centered by circumstance. (I hope that made sense). When there's no one around to talk to or about but yourself, then what else do you have to talk about?
Sorry this was so long, but I hope it helps.