Hello,
I have a daughter who will turn five at the end of this month. She's a happy child, but sensitive; that's a little girl for you. :)
She really enjoyed preschool this year, her little friends, dancing lessons and piano lessons. I try to praise her when she accomplishes things in school, at home or in dancing, etc. We still had issues to work through, which was normal. When she would get frustrated or say, "I can't do this; it's too hard", I would have to say many times, "Practicing homework or piano makes you a better student and once you learn a new skill it's fun to practice or demonstrate what you have learned."
I would invite her little friends over to play, which she really looked forward to. At this age, little ones are learning so much about themselves, others and their emotions. One day, she's mad at a friend, the next day, they're friends once again. They're learning how to share, cooperate with others, how to handle their emotions and independence.
Your daughter may find great satisfaction in something she enjoys, like dancing, music, art or a sport. Learning a new skill that's enjoyable and that she can do well will boost her self-worth, relieve stress and may help her adjust more easily in school. Also, music (singing, dancing, or playing the piano) can stimulate a child in learning math and literacy skills in school. Someone had mentioned the Kindermusik program as being beneficial. I agree; my children have been involved in Kindermusik for several years and we LOVE it!
It's a good idea to point out all of your daughter's strengths or what you love about her, very often. Positive words will put a smile on her face and a spring in her step. Also, set aside time to play a game with her or bake cookies. Read and snuggle with her every day or at bedtime each night. Tell her you love her and give her lots of hugs...every day. She can also be a wonderful big sister and help you entertain or take care of baby brother. I'm sure it has been a big adjustment for your daughter learning to share your attention with her new brother. I remember when my son was born, my daughter felt left out and envious of her baby brother for a while until the family settled back into a normal routine.
Another good idea is to read some sweet stories or fairy tales to your daughter that build self-esteem or teach a moral lesson. Ask your local librarian for suggestions. I came across a book that looked good titled The Star Catchers: Stories for You to Read to Your Child to Encourage Calm, Confidence And Creativity by Joyce Dunbar.
I wish you the best! I'm sure everything will work out just fine.