Scubanurse,
Relax, you'll be fine. Everyone (pretty much) experiences first time jitters and anxiety - I know I did. I almost didn't even do my OW dives I was so nervous. Once I got in the water, I was fine. Just take it slow...go down a few feet, then a few more.
Hey, I just did one of my first dives alone since getting certified and had an anxiety attack (vertigo) myself - so youre not alone. It happens. It happened to 2 of the 3 of us diving 2 wks ago. Just go slow and take your time. If you start to get scared, just back off, try and breathe and relax. Doing your open water dives is just like the pool, pretty much. In fact, its just more open. Water is water. You know your skills. If not, perhaps an additional class or 2 in the pool will help if you work on these problem skills - if any.
If your mask is leaking, find out why. Poor fit? Hair or a hood under part of the mask skirt? There is nothing more aggravating than a leaky mask. If thats a problem, fix it now or get a new mask that fits correctly! THATS important!
A Divemaster told one of the students in our class who was suffering from anxiety that perhaps they go talk to their doctor about a low dosage of Xanex (say .025 or less) before their dives. They did, and were fine. It was just enough to 'take the edge' off of their nerves, w/o making them sleepy or tired, etc. I am not a big advocate of drugs or alcohol before or during diving - but this person NEEDED to calm down a little - and that applied to their life overall and not JUST diving. Made a world of difference for them. Once they DID get in the water during checkouts, they had a ball and we all dove the 2nd day to 44 ft for like 30 mins, and they were fine and we all had so much fun. Just take your time, talk to your instructor and he/she will give you extra attention and help. They know - they have seen this all before.
No one is born like a fish. We live our life on land; don't breathe thru a regulator all day long, so this is all new.
Relax, keep at it, and practice makes perfect! Nothing comes easy - including diving skills!
Good luck!