So I know it may sound a bit weird, but I like milky tea with some lemon juice added. I pop a teabag into the mug, fill it about 2/3 with boiling water, let seep a bit, remove teabag, add some freshly-squeezed lemon, then slowly add cold fat-free milk (and add stevia for sweetness). How can I make myself a cuppa without the milk 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)—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.
I do this too.
I add lemon, plus I add honey to the water first before pouring it over the tea. Sometimes a smidgen of sugar. The sugar and honey help take away some of the acidity but it has to be in the water first and mixed up.
Then steep the tea.
Then after the tea is made and steeped, I'll add a little bit of milk.
**Also if you put a lot of lemon in it, this won't matter. How much lemon can affect this. I normally don't like the way it tastes if there's too much in it and too much tends to curdle the milk regardless.