A couple of things can counter mara jade.

Stress sinks like Istbam, Farlander, Tycho, or Jek who can get rid of stress or become better with stress.

Self stress abilities, such as R3-A2 or Jan Ors. R3-A2 works great once you realize that Mara Jade cannot double stress. So use R3-A2 to stress Decimator Mara if at range 1 as you are going to get stressed anyways.

Mara Jade is not an upgrade that you should have to build a list specifically to counter (or even have to think about at all when building a list) any good list should be able to beat Mara Jade with decent flying and strategy.

There's not point in making more suggestions until OP responds with his list and his opponent's list so we can see what the actual problem is, because I suspect the reason OP is losing isn't actually Mara Jade, it's some other problem with his list or how he's flying that is making him lose.