every, each
每 centers on the idea of repetition and distribution: something that applies to every instance or each member of a group.
每 is the older form of 毎. It originally depicted a mother (母) with a hairpin or ornament on top, symbolizing a woman who has given birth to many children, hence the idea of 'every' or 'each'. The modern simplified form 毎 reduces the top to a single dot.
The bottom part is 母 (mother). Imagine a mother who gives something to every child, each one in turn. The top stroke is like a mark she makes to count each child, ensuring every one is included.
For マイ, picture a mother saying 'my' children: 'my' sounds like マイ, and she cares for every one of them.