17, pregnant, and maybe depressed

in february i got pregnant and didnt find out til june. ive always been the really good girl thats the most responsible and mature, so because of that i hid my pregnancy from my mom scared of what she would say or do. of course she was really mad and said things she regrets but now shes the most support i have besides my adoption agent. i now have my family and it feels like everything is going to be working out. but lately its been so hard to just get through the day. im currently doing online school so i just sit in the house all day everyday not knowing what to do or think. lately i just want to cry all the time for no reason. my mom thinks im going back on my adoption but im not. i know this is whats best and what i should do. next week i have an ultrasound and im scared to death. ill be seeing the baby for the first time and finding out what the sex is and i know itll break my heart. My mom doesnt know what to do to make me feel ok, i dont blame her, shes raised kids for the last 17 years. i know im getting depressed and i dont understand why because i know giving my baby to a couple who can raise it ten times better than me is the best i could possibly do. i feel like i should be excited, my car is getting fixed, im finishing my senior year in a few months and starting college classes here soon. but instead of being excited i feel sad all the time. i dont know what to do anymore. i dont even know if this is normal. help?