Mozzarella curds turned sour in the fridge?

[TheGrateWall]

I'm brand new to the forum and to cheese making as a whole, and from what seems like endless stories, I chose the hardest cheese to make in the form of the 30 Minute Mozzarella Recipe using citric acid from Gavin Webber Here: https://www.youtube.com/watch?v=4VbuBcNCgAc

After 3 Attempts using standard Non-UHT pasteurized storebought homogenized whole milk, I finally got a ball at the end that I could work and shape. Unfortunately, I wasn't able to get it to stretch right (although it did taste somewhat like mozzarella) and I can understand that after reading countless posts with the same issue. It was probably a PH issue as I don't have a way to check that besides the stretch test which was never satisfactory. Anyway, I decided to try to store the "mozzarella" balls, and just 5-6 hours later, the "mozzarella" balls have turned sour in the fridge! They have a stiff, rubbery, slightly grainy consistency, and the curds taste really sour and just off.

I'm thinking maybe I didn't add enough salt and it kept acidifying in the fridge, or perhaps the milk was already going bad despite just buying it and looking for the freshest date.
Obviously, I was not successful in making proper mozzarella evidenced by the picture, but I didn't expect the curds to sour so fast and thought maybe I could salvage something out of them. As I said, I'm brand new to the whole process, So if you see anything obviously wrong then let me know!

Here my personal recipe I follow based on the 2 other unsuccessful attempts/Gavin Webber Video:

Ingredients
1 Gallon Great Value Whole Non-UHT Milk
1 ½ tsp citric acid in ½ cup of bottle water
1 tsp kosher salt
¼  tsp of liquid rennet with ¼ cup of bottled water
1/8th a tsp of lipase with ¼ cup of bottle water (sit for 20 minutes beforehand)
¼ tsp of calcium chloride in ¼ cup of bottled water (with homogenized milk, may prevent Stretching)

Steps: 
Make Lipase Mixture to sit
Put milk in pot and heat to 55F
Add Lipase/Calcium Chloride/Citric acid
Mix and heat to 90F
Add in Rennet (while stirring, Wash cup with milk) and Stir for 1 minute
Take off heat
Leave to set for roughly 5-15 minutes
Check for a clean break
Reapply heat
Cut Curd into roughly square inch pieces
Slowly Heat to 105F
Turn off heat and let cook for about 15-20 minutes
Drain Through Butter muslin/cheesecloth colander after curds cooked
Gravity Drain through Cloth
Add to a glass bowl and drain as much whey as you can
Put curds into the microwave (1000 watt) for 1 minute
Drain Whey again with fingers/gentle knead. Expel as much whey as you can
When no more whey comes, Put back into the microwave for 35 seconds (1000 Watt)
Knead and drain whey again (Use Rubber gloves if needed)
Should start forming a clump, continue to expel whey
Put into microwave for 30-35 seconds (1000 watt)
Add a tsp of kosher cheese salt
Begin Kneading, and start stretching once the salt is dissolved (may need to heat more if does not stretch) Don't Overwork it, shiny surface is the aim *Can be put back into the microwave for 10 seconds if needed to heat curd. Work swiftly
Make shiny balls and drop them into ice water
Let cool for 5 minutes, and store in a plastic or glass container
Hopefully Success (Obviously Not)

Also, thinking about trying something else to try to bolster my confidence and revisiting it as I'm feeling pretty discouraged currently. Any suggestions on some ACTUAL beginner cheeses? Very interested in semi-soft cheese using a cheese press, but fresher cheese is also very attractive since they don't require many extra tools. I'd be interested in whatever helps me learn. 
Thanks!