Tuning Gearbox selector not moving to far right

[YAMsThaBoii]

Hey guys

I'm driving a Fiat siena 1.2 El 2002/ 2003. I have a problem with my gearbox, im wondering if anyone could have knowledge of what exactly might be wrong, it's something I have to elaborate

My transmission/ gearbox won't move to far right side when I move the gear to select 5th/ reverse, basically the cable responsible for manipulating the gear selector or whatever only moves far left and center. Meaning I can select all first 4 gears but can't access the 5th and reverse. Aive tried all adjustments. I've removed gear cable responsible for manipulating selector or whatever, left to right and tried moving it with my hands, only far left side responds, I push it left (access to 1st and 2nd gear) and it automatically returns to center (3rd and 4th) but when I push it to the far right, it doesnt push back to center/ respond (basically jst wiggles loos on far right).

So I'm wondering of anyone has had such a problem before, and is it something I can fix my self, or even possibley if I can fix my self, I'm a self thought mechanic, can overall a engine and such, jst never opened a gearbox, thanks in advance.