DaemonUmbra

In CMD

Type java -version please

These should not be all on the same line, pause especially. You need to hit enter between them.

Even with pause at the end?

Step 1 of the steps given, use the jar version of the installer, not the exe. This means you need to get the generic "installer" from the website, not the "Windows Installer"

Java will only use as much memory as you tell it to use, right now you aren't even giving it half a gig. Use the arguments described on this Minecraft Wiki page.

I'm pretty sure it's been discontinued while AMD push Radeon Settings. My desktop CPU doesn't have onboard graphics so I can't test it myself but I found this on the AMD forums.

Don't add the [ ], those were just there in the instructions to signify that the author didn't mean literally type in "name of the installer"

You should not be inside of the jar, you should be in the folder that has the jar in it. And if it's Powershell and not CMD, before you continue with the rest of the instructions you will need to type cmd and press enter.

the jar

It means the name of the file, the way it shows up in file explorer

The jar installer is the one that just says installer on the files page.

The blank GUI was a misconfiguration that has been fixed in the most recent version. If you are running GUI-only, update and you should see the message that Lex has quoted.

You can create the folder yourself and put mods in or Forge will create the folder the next time it runs.

Yes... you need the vanilla Jar for Forge to work. Forge works by performing its modifications only in memory, not on the disk.

This usually helps

Put this 1.12.2 server jar in and try again.

You have old versions of at least 2 mods, try updating your mods and see if you still have this issue.

Are other mods working?

Try removing Instant Structures Mod

If it's spikes every few minutes, try lowering your RAM rather than raising it. Explanation: Java uses a Garbage collector to reclaim RAM that is no longer in use, giving Minecraft more ram is like increasing the size of your garbage can, sure you can go longer without taking out the trash but when you eventually do have to take it out the trash bag is now much heavier and it takes more work to take it out, too much ram will cause lag spikes when the garbage collector finally runs. The other issue is Forge does more than vanilla, so if you have a relatively low-powered laptop or something of that nature you may be stuck.

You will need to go to the website it gave you and look up how to uninstall Java 10 then.

Please provide your fml-client-latest.log as described in the EAQ and my signature below.

I'm not on a Mac, but Oracle has a .dmg available for Java 8 here.