be more generous

I was an only child growing up. (Later my Dad had 2 more kids but that’s another story.) I never really learned to share or to live communally, never had to I supose. All my adult life I’ve found it hard and felt bad and petty when I have these reactions.
It’s little things like sharing food from a communal bowl on the table. That always makes me back off, leave it for others, not like feeling entitled to take a share or even knowing how much is ok, relieved if someone else puts some on my plate. I just take as little as possible and try to get it over with quickly. Other things like putting stuff in the fridge and expecting it to still be there untouched later get to me as well. I hated being a student – all those labels in the fridge and then people just took your stuff anyway :-(. When it happens I feel thrown, resentful, like I said, petty. Like I’m still that little girl who never had to share.
It’s ok when there’s just two of us, I can cope with that but more and I get really uncomfortable. I suppose that’s why I usually serve food plated.
I don’t know what to do about it though.