Leveling out backyard pit - Sand + topsoil?

I had 2 little sheds where the base was getting eroded from improper drainage (close to house) and a groundhog made a home so we decided to just get rid of the sheds. There was a little "pit" maybe 2ft by 4dt and maybe about a 1 ft deep.

I filled it with just regular topsoil but noticed some "squishy" parts after some of this rain. After googling it says I should have mixed my topsoil with sand?

Is this what I should do? Just mix in some sand or do I have to dog out some of the soil and then mix it with sand and put it back?

Can I pick up this sand at home depot or lowes?

Thanks