Hey. I'm back again. So. I made a pizza and offered slices to my Mom. She rejected it and I ate the entire pizza myself. I feel guilty because as her daughter I wanted to do something nice and I don't know what to do.
I'll start broadly and then focus on your specific situation.
Altruism is being unselfish, altruism is putting someone else's needs before your own. If you choose to do something for someone else and your happiness depends on how they receive your act, it isn't altruism, it is a selfish act, because your focus is not on their need, but on how their response to your action makes you feel.

Identifying someone needs food -> making them food -> being happy that you did something nice for someone

Identifying someone needs food -> making them food -> being happy if they eat it, unhappy if they don't
The purest form of altruism is unconditional and anonymous.
So, to your specific situation:
You did something nice! You should be happy. The action you took was nice, maybe it wasn't exactly what she wanted, but that's okay, you tried, and now you know a little more about how you can be helpful next time. You just need to untangle your own happiness from the outcome. The worst possible result of this experience is that you remember how unhappy this attempt made you feel, and so you never try again.
The description you have provided ("she used to always put herself first before her kids") sounds like she may have spent a long time in poverty, sacrificing her own food needs for her children's. Even if you're no longer in poverty, it can be extremely difficult to shed the baggage, a lot of people who have experienced poverty will continue to live as if they are in poverty
even if they become rich.
It is quite possible that your mother simply cannot even imagine accepting food from her children because it feels like taking food off her children's plates. She may also have spent so much time going without food that she doesn't enjoy it, and is comfortable with her current diet, even if it is less food than she could have.
Ultimately, there is no quick fix, but you can try to create an environment where your mother feels more comfortable accepting food from you. For example, you might say that you want to have dinner with her, buy some ingredients and ask her to teach you to make something she has made for you before, something that is made as individual portions, so you can make one for her and one for you, and because she has shared in the preparation and knows you have enough food, she may feel comfortable accepting it.
Alternatively, you can also consider whether or not food is the right way to show her that you care. Are there any other things that she does like, or that she might find helpful? Does she love a specific type of TV show, or have a favourite band? You could do some research and find things you think she might like that she isn't aware of. Or, more specific to the household, it could be something like doing the laundry, or cleaning a room, or taking a sibling to school or... so on and so forth.
In summary: find things you can do for her that you think will make her life a little better. If a thing doesn't make her life a little better, that's okay, no need to feel bad, scratch it off the list and try again tomorrow with a new idea

A home filled with people who try their best to make each other's lives easier will be a happier home, even if sometimes those things are the wrong things.