I know many benefits of tea, and I have tried hard to drink tea, but my stomach does not fit inapprove of it.

Present perfect "I have tried" seems to fit best here; I suppose because, while the attempt is in the past, the consequences of it are in the present. In the past, you tried drinking tea, and that past experience is why you know, in the present, that you can't drink it. You have tried drinking tea.

"My stomach does not fit in it" - this makes very little sense. It sounds like you are taking your stomach out of your body and putting it in a teacup, and the teacup is too small to contain your stomach. You could maybe say something like "It doesn't fit in my stomach", which would make more sense but sound a bit odd. I would do something like this. My stomach doesn't like it, doesn't approve, my stomach has problems with it.