Why is my bread tasting too sour

I got a starter from a coworker and have tried to make bread twice. The first time I kept the starter in the fridge while I was feeding it once a week. After I made the first bread and tasted how sour it was, I thought maybe I needed to feed the starter more often and kept it on the bench while feeding it everyday.

On Friday I fed the starter before going to sleep so it was ready to bake when I woke up, it had more than doubled in size and everything looked good. I followed a recipe with 200g milk,200g water, 500g flour, 100g starter and 10g salt. I mixed it all together and let it rest for 30minutes . Every 30-45 minutes for the next four hours I stretched and folded the dough. Then I let it rest 2-3 more hours before I put it in the fridge overnight (16hours).

I have tried to do some research on how to balance the sour taste but feel more confused than ever , do you have any suggestions?