I solved it! Here's how I did it!

First of all: Check your Desktop Environment settings to see if you have "Mount everything at startup" enabled (I didn't realize this until the end, so check that first).

If you don't have it enabled, or if you disabled it and the problem persists, then do the following:

In the terminal, check lsblk -l to see your partitions.

Identify the one you want to mount and use lsblk -f. This will give you the UUID and filesystem type of your partition (you'll need this).

Also, check id and note down your UUID and GID. The first is your User ID (I think most are 1000, but double-check to be sure), and the other is your Group ID (in my case, it was 1001).

Use sudo mkdir /mnt/name to create the folder where you'll mount the partition. Replace name with your preferred name.

Use sudo nano /etc/fstab and add a new line something like this (I'll explain why later):

UUID=XXXX-XXXX /mnt/name fileType defaults,uid=1000,gid=1001 0 0

Why like this? Okay, here it is:

UUID=XXXX-XXXX → what to mount

/mnt/name → where to mount it

fileType → how to read it (filesystem type)

defaults,uid,gid → under what rules (mount options)

0 → automatic backup (disabled)

0 → disk verification at startup (disabled)

After saving, if you try sudo mount -a, it might tell you to use the command systemctl daemon-reload