Q.Monstera Deliciousa slowly yellowing
For this past Mother’s Day I received a beautiful Monstera deliciousa and I had it in a specific corner in my bedroom for about a month (maybe a little less). But the plant was a little too big for me in that corner so I moved it in a different area of my house. After about 2 weeks, I noticed a bunch of gnats around it, and the newly bloomed leaves were dying off. I also noticed that I only watered the plant once in about a month, so I figured the soil was just bad soil and the plant might be rotting (it wasn’t the only one, I’ve lost another plant to this soil). So I rinsed off the roots, cut roots that looked bad, got a smaller pot (since I lost a lot of the plant, I decided to put it in an 8-inch pot instead of the 11-inch one it was in). Used a better brand of soil and added little rocks to it so it now drains really well. I even put it back in the corner of my bedroom where it seemed to thrive in for a month. It’s been there for 2 weeks now and, one by one, leaves are turning yellow. Some of the leaves I can pull directly out of the soil because where the stem sets in the pot, and seems to have rotted. I mist it every other day, and it’s next to a window I keep open at least 3-7 hours a day. I’ve read the plant doesn’t like to be moved around and has an attitude, but I’m still not exactly sure what I’m doing wrong. What used to be a huge and beautiful plant now has 11 leaves. How do I save my plant?

Your plant looks actually pretty nice, but if it has been declining, it isn't happy.
Monstera need a bright location, but indirect sun. Direct sun will burn the plant. Elevating the plant from the floor may help.
I would also place the pot on a tray of water with pebbles for added humidity.
Make sure the top inch of so of soil is dry then water completely, making sure the entire root system is watered.
Here is a link with more information.
https://www.gardeningknowhow.com/houseplants/swiss-cheese-plant/repotting-cheese-plants.htm