When assembling a grip shifter, I prefer to sit the spring in the base of the shifter ( the opposite piece from your photo) and VERY GENTLY 'shake' the two pieces together with the cable fitted. Normally the shifter doesn't feel right till I have it attached to the bars with the cable under tension. A bit of trial and error will get it sitting properly. I have broken one of these before by being forceful and impatient, so be careful!