Here, but there is a common expression 'I am there for you' meaning that I will help you, support you, look after you, so it would OK to say: '"I know that you're always there for me.'

I have the CELTA qualification to teach, but I am not an italki teacher.

I know that you're always here with me. I know that you're always there for me. "with" means together, so if you are together, you have to be in the same place at the same time, so you should use "here" when you use "with".

It should be "here." "I know that you're always here with me." Perhaps your confusion comes from this: "I know that you're always there." It's similar. The first one shows confidence that someone is always nearby. The second, that the person is reliable. A variation on the second: "I know that you are always there for me." Yes, prepositions are confusing! This one means that you have confidence that someone is always ready to help. If I gave too much, the first line is my answer. :-)