I'm doing both Ancient and Physics at the moment and I can tell you that Physics is one of the most interesting subjects you can do but it's also one of the most complicated. If you are doubting yourself the safest answer would be to do Ancient History because it is just as interesting and involves the some of the same thought processes but is less demanding.
I also think studying a history is wise because science/maths student types can suffer from a severe condition informally known as "The Twilight Zone" (Doo doo DOo DOO doo dOO doo DOO). But seriously too many sciences and lots of maths can warp reality a bit, you lose empathy skills and tend to get a bit lost when someone mentions
morals.
Think of doing a humanities subject as a your tether to 'humanity'.