Hot tea with lemon juice and fat free milk: how to prevent curdling?
I don't think this is practically possible with lemon juice, as milk protein will curdle in the presence of acid (in this case, citric acid in the lemon juice)&mdash;and the process happens much more quickly at hotter temperatures, as in tea.

A pinch of salt may slow the process, but is likely to be unpalatable.

You could neutralize the acid with baking powder, but getting the right balance would be difficult, and again, the result is likely to be disconcerting and unpalatable.

If it is not the acidic balance that you appreciate in your tea, but rather the flavor of the lemon, you might try making a simple syrup with lemon zest, and using that to sweeten your tea.  It will give the lemon flavor, without the acidity, and so not trigger the curdling.