I can relate to this. I am 29 in a few days, very petite and no boobs and feel the way you do. I also get told that I should feel grateful I am so slim and tiny but like you, I feel really self conscious of my lack of chest and feel like I have the body of a 12 year old.
I have no proper advice as I am in the same situation but I wanted to reach out and tell you that you are not alone with feeling how you do. I look forward to catching up with this thread to see if anyone has some advice to give.