Try thawing frozen bm in the fridge first and chill the fresh one at the same temp. BM from different pumping sessions can only be mixed at the same temperature otherwise taste changes and risk of bacteria too. Frozen bm when thawed may have diff taste too due to high lipase. Check out this link: https://www.bumblebabychicago.com/lipase
did you freeze immediately after pumping out ? you can try 20ml and 80ml first slowly add.
Mummy J