My cooking is always flavourless?

I would look at your cooking technique. Some things loose flavour if over cooked and others need a longer slower cook for the flavours to develop. For example I would cook a tomato based sauce for at least 45mins to have any flavour in it at all, yet you say you add the tomatoes at the end. I think you are not allowing time for the flavours to develop properly. You need to cook things like tomato sauce for a lot longer and a lot slower.