Plan a time that you can be with him alone to discuss your past birthday. Wherever you decide to meet -- whether at home or at a public place -- make sure you are free from distractions. You'll want to give him your full attention and you'll want his full attention on you.
Express your feelings of hurt or inquire about why he forgot your birthday. He might not have known your specific birthday, especially if the relationship is fairly new; therefore ask him about it instead of making him guess the reason you are mad. Use "I" statements instead of "you" statements to avoid turning the discussion into a blame-game. For example, say, "I'm feeling hurt that my birthday didn't seem important," instead of shouting, "You forgot my birthday!"
Listen to him explain his reason for forgetting your big day. Stay calm and avoid getting angry and lashing out at him before he has a chance to explain. Count to 10 when you feel angry or the need to lash out at him, recommends Resilient Youth, a website dedicated to helping young people.
Talk to him about a solution and express your hurt feelings to him. Ask him if he has any suggestions for making up for the birthday, or if he is interested in re-doing your birthday. Now you get to celebrate your special day twice.
Plan a makeup birthday and ask your boyfriend to help plan the day. Tell him what you want to do for your birthday.