During the pool classes, I had no issues at all with mask skills. I could clear no problem. I could remove and replace. I could swim across the pool without it. But when I got to open water, I freaked out. . . . I haven't yet taken my mask off and replaced it without an instructor. I can't bring myself to try that again yet. I know it is rare, but I'm terrified of losing my mask.
Three thoughts come to mind: 1. You are not alone. If there is one particular skill or set of skills that I have seen present the most challenge to OW students (and some newer OW divers) it is the flooded / removed mask skills; 2. I am intrigued by the disconnect between the pool and OW experiences. Out of curiosity, was there a big difference in water temperature? I find that cold water (which is often the situation during the OW dives) presents a bigger challenge for no mask breathing than warm water; 3. Comfortable, effective no mask breathing is very much dependent on good airway control. And, airway control is without question a developed / practiced skill. For a few it comes naturally, or at least they don't have to think about it, But for many, it is a skill developed through application of technique, AND through regular practice. You have received a lot of good advice of technique. If you continue to work at it, in small steps, you can develop airway control and you will find your anxiety will dissipate. It may not happen immediately, it may take months. But, you
can do it with practice.
If my mask were kicked off, is there any reason I can't just hold my nose? Water up the nose seems to be the only issue that is presented by not having a mask on. I know it's not ideal, but while I try to recover my mask from wherever it floated off too (if I can recover it), or while surfacing because I no longer have a mask- is it possible? They didn't let us hold our noses in class, but there wasn't an explanation for why. I mean, is there an actual scientific reason like "your ears will blow up"?
There is no physiologic reason that you cannot hold your nose. In the OW course, we do not want students to hold their nose because it may undermine the development of airway control. Holding your nose is also not ideal in a 'real life' situation, because it occupies one of your hands, which you may need for another action (if you lose your mask and begin an ascent to end the dive, your left hand is occupied by your low pressure inflator, and if your right hand is squeezing your nostrils, you have no hand available to do anything else (grip an ascent line, access your spare mask, etc.) But, you can hold your nose if absolutely required in a real life, mask loss situation. (A similar situation exists with the regulator recovery skill. If the second stage comes out of your mouth, there is no reason that you cannot immediately deploy your octo and breath from that while recovering your primary. But, we have students practice recovering their primary without going to the octo, to help them develop the skill, and confidence in their ability to recover the primary.)
I cannot speak for the recommended techniques of other agencies, but the PADI recommendation (in the Guide to Teaching) is to have students learn to inhale through their mouth and exhale through their nose. That is what you were exposed to in your training. Ultimately, that is the degree of airway control that I would like to have all divers develop. It may be, however, that the small, incremental steps necessary to help a diver achieve that level of skill include starting with inhalation and exhalation through the mouth, after which they move to inhalation through the mouth, and exhalation through the nose.
If you can get into a pool, by all means do so, and practice while standing in shallow water, or kneel / lay on the bottom in shallow water, take your mask off and practice both methods (exhalation through the mouth, and through the nose). This can be done standing in the water with just a snorkel as several have mentioned. When done with scuba gear, one thing that I find is helpful to some is to tilt the head slightly to one side, to keep the bubbles - coming either out of the exhaust T on the second stage, or from the nose - from tickling you nose / face / eyes and prompting a desire to inhale through the nostrils. Since your troubles were in OW more than the pool, if you can go to an OW site with a shallow entry area, and do the same - in scuba gear, take your mask off and lay on the bottom and breath, then perhaps swim around a bit - it may help. Or, just stand in shallow water with a snorkel and put you face in the water and breathe.
I can empathize to an extent, because of my own experience during tec training. We had to do out of air, no mask swimming drills. At depth, we would remove our mask, signal out of air, and then swim on a donated second stage while being guided by our buddy. We did the drills in a cold, dark quarry (water temps at 50 feet were in the high 40s) and that skill practice produced the most stress of any scuba skill I have ever learned. I absolutely hated breathing without a mask in very cold water. And, while I was able meet the performance requirements, I won't say I was very comfortable. Recognizing that situation was a potential safety issue, and that I needed to continue to refine the skill after tec certification, I now force myself to go to a local quarry and practice cold water, no mask breathing regularly - I drop to a platform at 90 feet, where the water is in the low 40s, and take my mask off and breathe for 5-10 minutes, then put the mask back on. It was not pleasant at first (for that matter, it is still not 'pleasant'), but over a period of a year or more I developed the ability to breath without a mask in very cold water without anxiety.
Try some of the techniques that have been recommended, go to an OW site and practice in shallow water - if you get water up your nose at first, just stand up, clear your nose and start again. It may take time but it will be worth it.