The harsh truth on this one is your opponent overran you because you gave him time to rebuild. He turned the tables by slowly bleeding your manpower with his mortars while he built an army twice the size of yours. If you'd kept up the pressure he'd never have been able to do that.
BUILD
You made some good choices and some bad ones. The mortar halftrack was a very good call. Your opponent went with a ridiculously support weapon-heavy build (nine weapon teams!) which the mortar HT could punish with near impunity. It might even have been worth getting a second mortar HT in this match.
On the other hand you heavily overinvested in AT. You built a StuG when your opponent had no vehicles and you already had a Panzer IV and a Panzershreck squad and therefore that StuG did very little. You then built a Pak which never had anything to shoot at.
Every investment in AT isn't an investment in AI. Your opponent invested heavily in infantry so the tanks you want are the Panzer IV, Ostwind and Brummbar. The StuG and Panther aren't going to contribute much with no vehicles to shoot. If you heavily invest in AT when your opponent hasn't invested in vehicles they can simply build anti-tank guns and more infantry instead, which is exactly what they did.
STRATEGY
The sad truth of this match is even with your overinvestment in anti-tank weapons you still had the tools on the field to push your opponent back to his base all along: the Panzer IV and the mortar halftrack. However, you were very passive with them. The Panzer IV did some work but spent a lot of its time at the back of your lines with the StuG doing nothing. The Mortar HT spent most of its time sitting in your base. When it did come out to play you left it at the front of your army with no support, allowing enemy AT guns and AT grenade-wielding Conscript squads to walk up to it uncontested.
The enemy AT guns aren't very mobile: your P4 should always have been at the opposite side of the map killing infantry models at every opportunity. If you keep it repaired it's not going to die before you can react and if you keep infantry squads near it they can protect it from any attempt to snare it.
The same goes for your mortar halftrack: have that thing barraging the enemy support weapons pretty much on cooldown. Treat it like a more mobile mortar squad: keep it close enough to the front lines for it to fire at things but make sure it has units nearby to protect it and pull it back if the enemy closes in.
A few other minor things:
- If you're going to build mines it's usually better to build Teller Mines. Any opponent who's going to walk into clearly signposted S-Mines is also going to lose vehicles to Tellers and losing vehicles hurts a lot more than five or so dead infantry models.
- Panzershrecks are a sidegrade, not an upgrade. If you buy Panzershrecks you turn that squad into a dedicated AT squad. It's not going to win one on one fights.
- Avoid building units your opponent's build counters if you can. It's easy to slip into habit and always build the same things but you can gain a substantial advantage by adapting to your opponent. Your opponent built three mortars: that means you can't use your own weapon teams aggressively. Therefore use the weapon teams you have much more defensively and don't build more unless you truly need them. Go for the units your opponent's army will have the hardest time dealing with.