Breathing Issues- Is this all anxiety ?

Hi, I am a "healthy" 19 year old girl. I have been to plenty doctors and they all tell me I am fine. Except for the fact that they think i have bad anxiety along with bad allergies and asthma. I have been concerned because I feel like I can never belly breathe. I don't know if it is because I am thinking about it but I get so scared that I will just stop breathing. Sometimes i wake up in the middle of the night feeling like I'm not breathign and like I will die. This is SO scary. I feel like there is pressure on my stomach sometimes that is keeping me from breathing normally. Sometimes I even have ribcage pain. I even feel shaky when i really try to focus on belly breathing. Is this all anxiety or what could it be?! Im terrified and shouldn't have to be so scared at a young age! I'm supposed to be enjoying life!!! PLEASE HELP
Thank you