I was OW certified this past June, and I was apprehensive as well. My sinuses were damaged due to a horse kick to my right cheek bone years ago and I have had severe problems with them. I just knew that they would give me trouble.
During the pool session, I did fine and equalized easily. I had trouble in the open water section, but that was due to inflamed sinuses from water skiing the week before and not treating the inflamation prior to diving.
I thought that my sinuses would be my only problem, but I was wrong. Getting used to breathing underwater was a challenge that I hadn't foreseen. It just didn't feel natural. As much as I wanted it to feel like second nature, it just didn't. For 47 years, I had been blessed with all the air I could ever need, and while underwater, I didn't feel like I was getting enough. Having to pull air through a mouthpiece with my nose covered by a mask just didn't feel natural, because it wasn't.
It didn't help that my BCD wasn't fitting right, and my weights were too far forward causing me too struggle with my balance either. The more I struggled, the harder I sucked on my air, making it feel like my air supply wasn't adequate. It was frustrating. After the first 15 minutes in the pool, I wouldn't have given anyone a plug nickel for anything to do with diving. I just didn't like it. BUT I DIDN'T GIVE UP!! I talked to my instructors, and told them what I was feeling. They got my BC straightened out, my weights repositioned so I could get comfortable and help me to RELAX
Once I felt comfortable, then the breathing started taking care of itself. The less I struggled, the less air I needed, so I wasn't puffing on my resperator and feeling that my supply air was inadequate. I started having confidence in my ability, and my equipment. From that point on, I got more and more comfortable until at the end of the first day, I really hated coming up from the bottom of the pool.
Just try to get comfortable with your equipment and don't struggle with it. It will only complicate things. Once you learn to relax and trust your equipment, you should do fine.
Good luck on your dive. You've got a whole new world of adventure ahead of you!!