Update: Made bolognese with some of the tomato sauce. Added some tomato paste and whole milk along with pork and beef. The resulting sauce was silky and delicious. The tomato paste really helped add another layer of smooth rich tomato-y flavor. These tomatoes were end of the season, not exactly in their prime, but they contributed to a great sauce! Though I was at first disappointed about how the sauce turned out, now I'm looking forward to using the rest. Thanks to all again for your suggestions.

This won't help this time, but in the future, seeding the tomatoes makes for a less acidic (and less watery) sauce. Simply blanch tomatoes for 45 seconds to 1 minute, shock them in an ice bath, score the bottom of each to remove the peels, and squeeze gently over the sink -- the seeds will slip right out. The result is a thicker, less acidic tomato base.

go for the buttah- I had a very similar experience w a sauce this weekend and I tried a few tbsp of butter for the first time based on a recommendation from somewhere on here! It really balanced the flavors (I had 2 onions and 4 grated carrots in there already which didn't help- the butter brought it all together, seemed to reduce the acidity, and the sauce turned out great in the end).

If this helps.http://www.recipesecrets.net/forums/cooking-tips/32400-why-add-sugar-tomato-sauce.html

To balance acidity I use any one these honey, Sugar, carmelized onions. I have run into this with fresh tomato's but if a pinch of sugar is good enough for Escoffier then I suppose it would be OK.

More olive oil. And basil.

I gave up making tomato sauce on top of the stove long ago. I make it in the oven. I spread lots of tomatoes (any kind) in a big roasting pan (mine is nonstick), then sprinkle them with salt and pepper and add some olive oil, a handful of basil (if I have it), maybe an onion, and sometimes a tablespoon or two of brown sugar or a whole head of unpeeled garlic. I set the oven at 250 or so and forget about it for 6-8 hours. The main idea is that slowly roasting the tomatoes into a sauce seems to concentrate their sweetness. When the tomatoes are no longer liquid-y, I put them through a food mill. It's a wonderful recipe if you work all day, too.

Cook's Illustrated says to sweat the onions in butter. I haven't tried it this way yet because this was a lousy year for tomatoes here and I didn't get my usual bomper crop.

I'd go with what TiggyBee says about Simone Beck. A friend of Julia Child's can't be wrong, right?

Well, I suppose acidity is good for canning anyways. I can adjust later as pierino suggests and use the sauce for bolognese.

I've resorted to butter before. How did the tomatoes taste before? Maybe the problem started there.

a little heavy cream, or half-and-half

I know, right? I softened the onions and carrots too before adding the tomatoes. The garlic is fresh. The sauce is definitely not bitter, just tart. I've heard of adding dairy or butter but was wondering what else others might do.

I'm slightly baffled as the combination of onion and carrot should provide enough sweetness. Is the garlic old? That will add bitterness. The addition of sugar makes me cringe. Depending on how you are using your sauce, you might think about adding milk to it as in sauce Bolognese which would combine mixed meats with the ingredients you described----but I would only do that if you were preparing a sauce for pasta. My suggestion would be to leave it alone until you get closer to the finish line for your meal, and then figure out if you need dairy in there.

A pinch or 2 of instant ground coffee...this according to Simone Beck's book Simca's Cuisine. I've never tried it, but it's a helluva creative solution!! It's actually a cool book!