Tomatoes being too tart can sometimes be remedied by introducing a different kind of acid that takes precedence on the palate.
A teaspoon of cane vinegar, or white wine vinegar, or sometimes even cider vinegar can "replace" the acidic taste that you get from the tomatoes. You're also going to need some good salt to make it all come together.
What can sometimes happen, practically speaking, is you taste a hint of the vinegar along with the savory parts of the tomato, but don't taste the acrid acidity that some kinds of tomatoes have. That taste is still there, you've just replaced it with something milder, so it's not noticeable anymore.
I've used this trick with a bunch of different canned tomatoes that I bought while living in various places in S.E. Asia (it was hard to get good fresh or even canned tomatoes in some places) and it works ... 85 - 90% of the time.
You may (as others have pointed out) also find some luck with a little sugar. If you don't want to put refined sugar in your dish (which is a perfectly normal thing to not want to do) you can finely grate some carrot on a microplane and let it cook in.