My lasagna sauce is too tart when I first set the sauce to simmer, it's fine, but after simmering for 2 hrs. it's more tart than I like.  How can I counteract this tartness?
Typically, the longer tomatoes cook down, the more tart they become. You can add sugar to counter this. Add a small pinch at a time, tasting after each addition, until you have the taste you want. Remember, you can always add more but you can't take it out.