I don't want to split a sandwich with my then-gf even though we had agreed to go halfsies
Why do you want to go back on that agreement to go halfsies with her?
because at the beginning of our relationship I told her that she rarely shares anything
I understand that you might have some differences with your girlfriend, but you should honor your previous agreements, it is only fair.
It's because I don't want to split a sandwich, if she won't be happy about it we need to break up.
It seems like the easiest solution would be to order a second sandwich rather than potentially breaking up with your girlfriend over you not agreeing to go halfsies as promised.