Ok. I will split my answer in two sections:
1. How you avoid such situation.
1.1 You need a proper build up. You build infra till you switch to war eco. You do it with attache into spain and Turkey incident. Infra in Moscow, Leningrad, Stalingrad + Krasnodar if you want for extra tungsten in the future. Anyway, till you switch to war eco. Then, You build civs in high infra regions. Moscow, Stalingrad, Leningrad, Kiev., Krasnodar till mid 39. In sp is fine. You aim for 200 civs. Then you build mils behind the river line, in Ural and in Vladivostok to take tankograd. Overall, you have too little production.
1.2. There are things to work on with focus order. Third five year, if you can't go down and take -2%, Industrial modernization and so, on
is bad. Don't do it. Try to rush both research slots before the war. Dont send ultimatum to lithuania, because it extends the front line. bessarabia is also questionable, there are more useful focuses.
1.3. PP usage: go attache for spain, war eco as soon as possible. Don't waste pp till the civil war in Spain. Go civil construction guy and industry research. Other things - not so important.
I will stop here. There are too many things to cover. I just wrote those, which are essential for early game -> most impactful later.
Cloak71 makes very good HOI4 videos. Soviet Union guide is not an exception. Here are two videos. Watch them. The game vs AI without expert AI will become a total annihilation (not of you, of the Germany ofc).
2.
What to do now.
2.1 Go total mob, women in work force, War bonds, we will accomplish high yield. Spam last two on cd. Build mills, is fine.
2.2 Go gosproektstroy, take decision to invite american expects and this for minus Consumer goods Civs
2.3 research construction techs and resource extractions to be able to do resource decisions.
2.4. Switch to the proper tanks designs and division templates.
Infantry:
Since your industry is mediocre, it makes sense to use 21 width infantry. No need for AT actually, but if you insist you can leave it.
add one infantry to strelkovaya divisia. Wonderful.
Now to what matters -
Tanks. That it is where your main production should flow in.
Use 30 width tanks. 10/5 Tanks/Motorized or Mech is good. If you don't have enough tanks - Use something like 8/7. Stick to medium tanks for now.
Here is a tank design guide.
https://steamcommunity.com/sharedfiles/filedetails/?id=2714213712
try to max soft attack, to reability of 80-90%, enough armor to not get pierced, speed depending on your designing capabilities, for now not so important.
No need for other templates.
2.5. You have 3k planes in stockpiles. Use them. Fighters in groups of 100, Cas in groups of 1000. Deploy and train them till they are regular. As soon as they are trained, you are ready to strike. Try to have some (4-5-6) 30w tank divisions till then.
2.6. STRIKE BACK FOR MOTHER LAND:
With all your planes, four-five 30-width tanks we will strike back in the south. There are many plains there and the Germans are overextended. Better to do it in summer to avoid debuffs like mud or deep snow. Depending on how capable you are you should strike and encircle germans at the coast. Something like this:
View attachment 866056
All air force up, get the infantry into breakthrough. Encirle, squeeze the encirclement, take ports from them, go for Sevastopol after cutting them off. dont let them escape.
This should be enough to turn the tide.
I forgot: put maintenance in your tanks and logistics in all of your units: you should be using two templates. Infantry and tanks.
Good luck, comrade, the victory is near!
P.S. delete all cav and NKVD for their equipment. Stop heavy tank production, dedicate those for arty, aa, and mediums. I would also stop At and remove them from divisions. Get 40 mediums, you will have enough reliability with them to go for decent SA, armor and speed. You can go for 43 meds, they are also good and you won't take time penalty at this point. Create this mech variant and produce it.
View attachment 866062
Edit: typos