The easiest/cheapest fix is to replace your shower fixture with a temp sensing unit. $100 or so for a cheap unit and you're in business. It will cut back your water pressure but maintain the temp.

Thank you. Is there a way to maintain the pressure?

maintain pressure - yes - repipe...lower pressure and this hot/cold situation when someone flushes the toilet is because the pipe run to your shower was prob. done with 1/2" pipe....if it had been piped with 3/4" this usually doesn't happen...at least it doesn't happen in my house (I ran 3/4" to within 10' of every fixture and I experience no press. drop or hot/cold problems regardless of what gets turned on throughout the house).