If you're more an early aggressive kind of player, Lieutenant.
If you´re more defensive kind of player, Captain.
Else, it is difficult to tell you what you need to do from what you say. The only hint you give is that you struggle around mid game which let me suppose you have hard time to manage your medium tanks micro. Well if it is the case, you need to train more on this particular aspect.
About commanders, mechanized Commander requires you to be aggressive, here again it depends of you. Airborne is a good compromise between aggressive and defensive play. The early Pathfinder can provide you a lot if you understand it well how it works in term of damage output and vision. Infantry Commander is probably the best commander atm for defensive play, the one I use when I don't know exactly how to manage the game, if I see particular commanders and fear the use of specific abilities that counter well Airborne.
So here we go, are you more of a defensive or offensive player, or maybe both but it depends of the map or faction you're facing, or the position on the map or whatever...
When you know that, you can pick your officer and commander and develop your strategy.
i am typically a more aggressive player, because i feel like you have to be when playing as the allies vs the Germans. i subscribed to the believe that not giving the Germans room to breathe ever, helps keep them from massing tanks and inf blobs etc. sorry i didn't put a lot of details because i wanted to avoid falling into the pitfall of just complaining. typically i use rangers as a good shield assault force because of there beefy ness. if i have vehicle problems i tend to play airborne or mechanized , airborne because the bazookas vet well with paras and mechanized for the up gunned Sherman. over all i tend to shy away from Armour as the u.s because it feels like my inf response along with at guns is much more reliable to dealing the damper safely than a Sherman or Jackson which feels like a glass cannon.