It takes time for the body to accept smaller portions and healthier food as "normal". However, until it does, eating this way will feel challenging. My own body took 3 months to reach the point when I just felt too full to finish what had previously been a "normal" helping.
This taking time for the body to adapt to a NEW normal of smaller portions and healthier food is a big part of why successful weightloss doesn't happen quickly. Be patient - your body IS changing!