What are the options for sweetening?

Some quick things which you may or may not find in your nearby stores which can act as sweetners:

1. Honey
2. Jaggery
3. Corn Syrup
4. Some citrus fruits like oranges
5. Berries
6. Winter Squash like Acorn, Butternut

IMO, tomatoes should not be "bitter" it should become sweeter as it cooks down; usually we add some sugar to remove some of the acidity, but not the bitterness.

Use fresh, ripe and in season tomatoes; leave them out of the fridge.

I suggest slowly softening onions (up to before they turn brown) in oil (instead of boiling it); that should bring up some natural sweetness from the onions.

Add a little bit of tomato paste; this will add some sweetness (and more depth to the sauce).

Using dry herbs can render sauces/dishes bitter; use fresh if possible.

Added citrus will add some sweetness (oranges), but using the zest might also add some bitterness.

I would like to emphasize the sentiment given in the answer above that tomatoes shouldn't be bitter. If you are using canned tomatoes, consider moving to a different brand to give you a fresher and brighter tomato taste.

Having said that, I would also offer you another alternative called a gastrique to use as a way of finishing-off your tomato sauce. There are a ton of variations on the theme of the gastrique, but they all involve heating some sugar in about half its weight in water slowly over medium heat to caramelize the sugar slightly before adding something acidic like vinegar or citrus to the mix. For a gastrique that is more strongly acidic, sometimes the sugar is caramelized directly in vinegar to reduce the vinegar.

The vinegar and sugar gastrique is a common addition to tomato sauces. And while vinegar and sugar might seem to make strange bedfellows, their impact on the taste of tomato can be astounding.

Many times, just tossing a little sugar or other sweetener into the sauce can result in a sauce where you can taste both components but not necessarily an improved composite. Used conservatively, a gastrique tends to amplify what is already pleasant in the tomato sauce without announcing itself as a separate entity in the mix.