Anne,
The first step in overcoming anxiety, is to help determine what happened to bring it on. For example, is your anxiety a "trust again" issue, or a confidence issue. I'll give you an example for me...
When i'm training and playing around in a local pool, I do breath hold finning (underwater, usually hugging the bottom). The idea behind this isnt to see how long I can hold my breath as compared to others (perhaps going beyond my limitations in attempt to exercise bragging rights), but to practice a breath-held distance swim just incase something happened underwater and I needed to resurface without a working regulator. It's an exercise I highly recommend, but I wouldnt breath hold longer than 60-90 seconds. But onto my story ... as i'm reaching the other side (I swim deliberatly slow), I start getting a good urge to breathe, which is normal. But what isnt, is my mind now goes into a frenzy about this being how it feels to suffocate to death. Now strange as it sounds, thats when I have to really focus to keep my thoughts under control. Given the situation (no holdling upto or past blackout levels, shallow water, and reasonly good health and condition) I shouldnt be feeling this way at all. That type of anxiety prevents me from trying to further the distance, so it first must be overcame. So now, I need to figure out where this "thought" process originates from and how do I overcome it.
It mightve came from a bad swim lesson when I was younger, or growing up slightly obese perhaps from not being able to "catch my breath" as easily as others. It could also come from having the wind knocked out of me (gasping for air but nothing is there), or from a close family member dieing right before my eyes. It really could be any of the above. Since each is in a way an intangible but real problem, the only way to overcome it is this... set a known limitation for my distance to that which I know I can comfortably do. Which, over time will help to build my confidence on sucesses rather than failures.
If the anxiety is brought on by trust issues, then have confidence in yourself that you are a good person, and do make good decisions. That often we dont need to cast ourselves out into the blackness of blind trust, that many times we can and should only offer up that which we know and trust. For example, if someone says "Hey, let's go cave diving. Trust me, i'm the best". I'd have to reply, no thanks, you might be, but i'm not. There's nothing I can offer here, since i'm not certified, nor have any present desire to be certified as a cave diver. I'd be a liability, and a big one at that. On the other hand, if they were to say "Hey, there's a nice wreck dive just 20 miles out from shore, it's in shallow water not deeper than 45'. Interested?". Again, this is a more tempting offer since the depth would be within my breath-hold swimming range (pool training), but what would happened if we, for any reason, got 20 miles out with no way back. Anything I could bring to the party here? Common sense maybe (like, lets tell someone where were going), but physically no (if were stuck, I cant snap my fingers and whisk us away). It's exercising these "controls" over our decision making that helps us to gain confidence that we are making the best decisions, where we wont get into any trouble.
Good luck! I know you can overcome anything.
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Mike.