I'm playing Germany and am at war with the USSR
the USSR is currently the only threat
UK is subdues with constant IC bombings (he rarely even sends bombers to me), France is conquered and the USA is in the power of the German-American Bund and is only raiding my convoys (and is drifting towards axis)

The problem on my USSR front is that I'm pushing forward incredibly slowly
I've been at war for a whole winter and autumn, and I haven't even managed to push beyond Romania in the South, in the North I just pushed out of Estonia

The way I've been going about it is attacking where I outnumber, from multiple directions, move forward, and attack again, rinse repeat
I just don't know how I'm supposed to deal with the USSR's godawfully large number of Infantry though, I'm stretching my line thinner and thinner, but the USSR doesn't need to

Any advice on what I'm doing wrong?

P.S.
I could provide screenshots if needed

P.P.S.
How the fuck does naval combat work?