Its definitely your fin; its way too big.(I use a 38 on 7.5 and its fine for freeriding)
As a general rule multiply sail size by 5 and add 3. (so a 5metre would need, (5x5) +3 = 28cm..
Also move harness lines back; helps get rig forward. (ie CoE forward of cetre of resistance will make you bear away.)