How are you supposed to play this online?

[mrminiling]

You're painting a picture that you're an inexperience player.

If your vehicles are being knocked out by field guns, it is because you are being bested by ambush, or pushing into their LOS carelessly. If you know that they have anti-tank capabilities, such as a field gun, it is never good to push up armor unless you have an advantage; such as means (but not limit to) of being defilade from the enemy gun (read: to flank, to provide support as firing platform) or using concealment (i.e. fog of war) to use appropriate weapons to fire onto known AT position.

Being ambushed is the worse that can happen, and will happen. You can only react accordingly to pull out the best from the bad. But to minimize being ambushed is to minimize the chances of loss (element of surprise is a tough cookie) by using reconnaissance. Single scouts (any sort of infantry) sent out as a sentinel or recon will give priceless information about the enemies' position or attacking formation. Also bounding from cover to cover.

Anti-tank being fielded by infantry requires flanking and ambushing... go around, hide men in bushes, buildings. Running men up to attack a vehicle is not a strategy but a poor understanding of repelling armor.

A lot of things can be said to combat a defensive enemy or to defend, but it really is up to you as a tactician to formulate an appropriate means at the given scenario at hand.

As it has been said before, watch videos, I suggest commentaries (shoutcasts), on competitive play to understand ways to react and style of play.