You have issues, but nothing that can't be managed. Yes, you need to shift a portion of those troops northwest of Riga. No need for a frantic withdrawal, just pull back behing that Emajogi River and use the terrain, positioning units in the swamps, woods, etc. Run your line, basically east-west to the Valdai Hills, it doesn't have to be a big carpet. Move that green army of the Moscow MD to positions where you can slow his advance east. You cut his supply lines, so good work there. Send a reserve army if you have one to back up that Moscow MD army. And the terrain is very defendable. He might try to cut sharply north and cut off a huge # of troops so be thinking about that.
In the center, transfer troops there from Southern and Southwest Fronts if you have to, You're not under great pressure there and nothing of strategic value. I'd try and double up on the eastern bank of the Dnepr near Kiev, use the Z-key to help keep the combat values consistent and put another line or two behind that.
Have you loaded up those front line armies particulary in the center with RR construction brigades to build forts faster?