You can add some sugar to off set the bitterness. You can also try adding some dry seasonings - Italian blend or Mexican blend depending on how you plan to use it - to hide the flavor.

Dave

You could also add a spoon full of jelly. I like adding grape jelly/jam. I think it actually adds a little something in addition to the sweetness.

I've grown used to my own canned tomato sauce(Small Patch Preserves: chunky tomato basil sauce). Now the store bought sauces seem sickeningly sweet to me.

We make tomato sauce all the time with whole canned tomatoes from our garden, including the seeds, and I've never thought that it was bitter. However, we have noted a big difference in sweetness depending upon the variety of tomatoes used. The sweetest sauce we make comes from Super Marzano tomatoes. So maybe next year, try some different varieties?
Meanwhile, this website may offer a fix.

Here is a link that might be useful: Splendid Table

Many thanks to all. I added a little sugar, and a tiny amount of baking soda and that solved the problem. I will freeze it, and it should be yummy for pasta sauce for this winter!
Tomato-wise, this was made with a real mashup of various tomatoes that had gotten ahead of me. When I realized that we couldn't eat all of them fresh, I froze the surplus. Next time I will try to stick with paste tomatoes for sauce.