Tomato sauce tastes bitter

So, I have nine gallon bags of frozen tomatoes. I processed them with a food mill to remove skin and seeds, put them in two pots to be boiled down, and thickened them by boiling gently all day as to not accidentally burn the sauce. Upon tasting, I noticed it’s a bit too bitter. Did I do something wrong? I’m aware of adding sugar to alleviate this and the idea of acidity being a problem. I tested this with a cup and added sugar and a pinch of baking soda but alas the bitter note remained. All feedback is appreciated. submitted by /u/Dry_Cauliflower_3559