When a new baby joins a blended family, older kids can feel pushed aside. Saying that out loud is allowed.

Feeling edged out when a new child arrives does not make you ungrateful or unkind. Attention really has moved, and noticing that is accurate rather than selfish. Those feelings tend to get heavier when they are held in silence, because then you are managing the feeling and the hiding of it at the same time.

You could tell one person what this has been like for you, in whatever words you have. A parent, an older cousin, a teacher. If saying it to family feels like too much right now, that is fine too, and writing it down for yourself is a real start.