As an instructor I can say that it is not uncommon for people to have sticking points in the training. This is common in the learning of physical skills. This skill just has to be mastered, and it is an important one. Just think how well you will know how to do it when you finally succeed because it did not come easy.
You took the book learning seriously, and you are coming to dive peers for advice. You also toughed it out in the water when you where getting cold. Sounds like you want to do this diving thing. You must be in the right place. Learning individul skills takes as long as it takes. That is all there is to it.
Sometimes it is hard for an instructor to see what exactly you are not doing right, or to express directions/demonstrations in the magic way that clicks for you. He/she can not see inside your sinus cavity very easily if that is indeed the problem. It is likely that your instructor is getting a little frustrated too but with the problem itself not you personally.
My question is does the instructor demean you for this or encourage you to keep trying? I feel that positive reinforcement usually works better all around.
Without seeing you in action it is hard for anyone on the board to give you the exact direction that will resolve this issue.
I suspect the sinus problems played a role, and the added stress and frustration of not performing perfectly in what you percieve is a reasonable amount of time... First off it is okay and it does not directly represent how good of a diver you will be at the end of class. A reasonable amount of time is the amount of time it takes.
There were many good suggestions on this thread. I recommend you let your sinis problem clear up, read some of these posts, discuss your apprehensions and difficulty with your instructor, and above all relax. Do this before you get back in confined water if that is possible. If that does not work ask the diveshop if they can have another instructor that can help you out. We call that team teaching. Usually team teaching happens due to scheduling problems but if necessary it can be done otherwise.
I suspect your instructor really wants to help you succeed.
Don't give up. Look at all the divers on the board cheering you on. Diving is a great sport that will let you experience things not everyone does. Barring any medical issues, I think you will be a diver soon.
We (divers) are generally a good bunch, and most of us remember what it was like being in your fins taking class. Once you are certified, you will find on your dives that many divers are more than happy to give you tips when you ask for them.
Good luck
P.S.
If you have any questions about training/diving that you may be anxious/uncomfortable about asking the board, your instructor, or dive shop you can send me a private message, and I will do my best to help