"But I'm a shut-in, how do I even start to socialize again?"

Here's the trick - if you look at every solid friendship, it almost always started in one of two ways:
1] Network expansion - they met via friend of a friend. You make friends via your existing friends.
2] Groups. Workplace, lab, barracks, choir, practice session, class, running group, etc. A consistent place where you meet up with the same people repeatedly.

If you lack #1 you HAVE to get #2. There is no reliable third method to making new friends and getting a social group. You don't usually just walk outside and meet people and make friends - that can happen occasionally but it's rare and unreliable.

You also just need to stimulate your mind as much as possible. Read new books, listen to new music, pick up new hobbies. Do zone 2 cardio every day, outside. Run a different route so that youre getting different visual stimulation.