This is just a guess, but I imagine you're too heavy for just the tricycle gear setup you've got going on there...your plane is sagging towards the tail because there's nothing holding it up, buckling your aft gear inwards from the vertical in the process, causing camber (if that's the right term) in the aft gear, which is steering your plane. @keptin discusses this phenomemon towards the bottom his Basic Aircraft Design Explained Simply, with Pictures thread.
TL, DR: Another couple of sets of gear aft towards the center of the plane and you should be fine.
Can't tell where your CoM is - could you add some screenies with your CoL and CoM turned on, purdy please? Given your engine configuration, I'd also suggest turning on CoT; you might have problems there.
Typically, if your plane sinks once you're airborne, it's either because you're excessively stable (i.e. CoL too far aft of the CoM) or you haven't got sufficient lift (i.e. your main wings are too small). Looking at your plane, I'ma guessing a combo of the two. Could also be insufficient pitch authority as well, I suppose. Really need to see those markers to be sure.
