1) the brass ferule on the end of the cable housing (the end that goes into the fitting on the firewall) probably has a sharp edge, which will cause sticky operation if things are not perfectly aligned. To remove this burr, simply slide the cable out of the housing and take a drill bit (say, a 1/4") and just twist it in your fingers, into the end of that brass ferule. This will slightly chamfer that sharp edge, and help things slide more easily.

2) lie down on the floor and gun-sight the hole on the end of the throttle pedal rod with the hole in the firewall - if these are out of alignment the cable will struggle to slide freely. Bend the rod as necessary to get them to line up.

Hope this helps, along with the other folk's comments, Norm

You may be missing the throttle pedal stop that keeps the cable end of the pedal from hitting the firewall.

Once you get the cable routed properly and the return springs are in place, feed the end of the cable core through the hole in the clamping bit on the throttle.

Now pull on the core with your fingers (or pliers if needed) to pull the gas pedal up off the floor.  It doesn't need to be super-tight - you'll hear the pedal 'clunk' when it is up. Then tighten the clamping nut without moving the throttle lever - it should be down tight against the idle speed adjustment screw. When done, there should be a wee bit of slack to allow the idle speed to be turned down. To check, turn the idle screw to slow the idle speed - it should do so. If not, adjust the cable again, not pulling quite so hard.

When replacing throttle cables i usually find the bushing or ferrule through the firewall has a groove in it from the old cable so the cure is to driill it out slightly the next size up so the cable can slide freely, also check the the hole in the end of the accelerator pedal lines up with the hole in the firewall.

It sounds like both before and now you have some additional problems with the throttle cable and its routing.  For really helpful answers, post some additional information about what carb(s) you have and the age of your Mini.  If you are unsure, a couple of pictures will help.

The too-much/too-little throttle problem may be because the throttle return springs are missing or perhaps you have other issues with the linkage.  Again... pictures will help.

Me and my dad replaced the throttle cable on my classic mini, cause the gas pedal was too tough to press down on...                                      We installed the new one yesterday, the gas pedal isn't tough to press down on anymore.                                                                   However, now we can't figure out, how to align it properly... It's Either way too much gas or not enough.                                                      Do any of you guys have any tips or advise, thanks...