Have melasma on my face bad, use 15% vit. c and retin a, when is it best to use vit c serum, i use retin a at night, or do i use both together. tyvm?

Am: Retain a should be used at night, as you are doing. Vitamin c is best absorbed as the first thing that you put on your face after your shower. Always use sunscreen afterwards. You can also use it at night also if desired, but definitely in the morning. With melasma, there are commercial products known as "brighteners" which can also be of help to even skin tone.