Tomatoes are already acidic, consider non-lemony tomatoes next time if possible.

As for this batch, both sugar and baking soda works somewhat and you can to both.
A little at a time so you don’t change the taste too much.
Onions are acidic too, but much less so than lemontomatoes, adding a bunch of em to the sauce can help.
If you don’t plan on freezing or canning and you’re just making sauce to put on pasta or something, add cream to it to make a sauce rosée, it’ll mellow the perception of acidity a lot too.
Or use in it a chili where the beans, while not chemically buffering the acidity like baking soda would, can help absorb some of the taste.