I really struggle with this. (it means that it is quite difficult for you to do this, that you have to put effort in it to do it)

The literal meaning of this verb is to make an effort against some sort of restraint, or difficulty. It can be violent, i.e. a battle or a fight, or an everyday difficulty. In colloquial English we frequently use it to exaggerate how difficult something can be. 'I'm struggling a bit with this Maths problem', or 'he was struggling to come to terms with his divorce'. So, it can mean to fight against something, or to try very hard. Equivalent verbs in Portuguese would be 'lutar' or 'batalhar'.