Mixing foundations

So I bought the nyx total control drop foundation in shade deep rich and it was a little too dark for me so instead of buying a new one I decide to mix it with this lighter cover girl foundation I have (it’s a little lighter than my shade) whenever I apply it . Does anyone try doing this too? Like when I mix it it’s hard to get the right color, like it’ll look right at first but then when I cover my whole face I notice that it’s wrong. There is no real advice anyone can give me but who has experience with this? And any tips on matching foundation, application, etc in general.