So, I wanted to install some mods, so I got Forge 1.15.3 (or 1.15.2, I forget). I wanted TConstruct, which only worked with 1.12.2, so I installed Forge 1.12.2. It wouldn't work, so I deleted the other Forge (1.15), and it started working. I installed TConstruct, but no avail. It didn't show up in the 'mod' section of Forge 1.12.2. I did some research and figured out that you also needed Mantle. No matter what I do, I can't play TConstruct. I made sure I had the right version (Mantle said 1.12 in the file, but the description said it worked with 1.12.2. It's in the right location, I've uninstalled the mods and reinstalled them, and I've already searched the Internet. How can I fix my mods not showing up?

Note: I tried downloading OptiFine and it wouldn't show up either. I don't know, though if OptiFine would show in the first place.