Since they're at land tech 5 to your 4 they'll have better infantry than you, try focusing on getting to tech 5 and upgrading before you invade again. Get a general if good shock if you can. Use terrain map and find some hills, even better if BUR will need to cross a river to attack you, fort up there and see if BUR will attack you. Hopefully they will and you beat them, but if not you can siege the province. If attrition is a problem split your army, but make sure you keep an eye out and bring them together if BUR moves against you.