Personally, I don't use FAR. However, since it models aerodynamics more accurately than stock, you should be able to draw from RL planes and apply it to your craft.
Here's a possible solution (source):
It's hard to see in the image you provided but it looks like your craft has the shape of a triangle. According to the design proposed above, it should rather be V-shaped. A quick check in stock KSP: works like a charm
edit: maybe the airbrakes on my wingtips make the difference
Chemp's post in Making a real flying wing [FAR] was marked as the answer