Forge 1.8 won't work.

go to installations, then click new, then under version try to find forge. if it is there select it. If not then I can still help you but I need to know it's not there.

Using a custom launcher is not necessary if you don't want to use one. If you click the "edit" button on one of your profiles in the "installations" tab on the launcher, have you scrolled down to the very bottom of the "versions" list? Older versions of Forge appear at the bottom of the list rather than near the top.

As star_weaver says, if you are willing to try MultiMC, a different launcher for Minecraft - much easier to use for modding your minecraft versions. You can dowbload it from here :

https://multimc.org/

use multiMC because the mojang launcher is complete dog crap when it comes to modded