Terry covered many of the potential causes for anxiety attacks at depth, but another one may be gas. As an older guy who has been diving for awhile, I'm pointedly aware of the changes I've had to make simply to accomodate growing older.
Checked your profile, but didn't see an answer re training, so please excuse my questions - but are you breathing air or nitrox, or even adding helium at depths below 100 fsw?
Bad gas (e.g. with either carbon monoxide or oil vapor in it) is always a problem, but sometimes changing from air to mixed gas can help reduce anxiety. Adding helium to a mix intended for use below 100' can reduce potential for narcosis, for example - and narcosis can have all sorts of anxiety-provoking effects.
Other than that, I've had drysuits and hoods that have 'shrunk' over a decade, with the result that I can feel more constrained inside them; if you're overweight and out of shape it can play a role in comfort; if you've started taking maintenance meds for hyper-tension, high cholesteral, or cardio-issues that can also play a role. Physical and medical issues can contribute to psychological effects - such as anxiety at depth.
Equipment issues can certainly play a part, but if the equipment has worked fine for many years, unless there has been a sudden change to your equipment - it probably isn't the equipment. Simple to test - try a dive with a different reg. (Although high-performance regs are simply much easier breathing at deeper depths - but so is using helium in the mix.) I suggest you consider other potential causes besides just your regulators.
(Have you had a physical exam recently? Something to consider...)
Best with it.
Doc
Yeah...I've thought of the growing older angle as well. My dry suit as well as most of my wet suits have "shrunk" over the years....heaven forbid that I admit that my belly has gotten larger Just for the record I'm a NAUI Master Diver...which makes my condition all the more frustrating.
During the dives I've spoken of, I've always been on nitrox. However, recently (last weekend) I dove on regular air and it didn't make any difference about my mental anxiety.
Actually, I started out the weekend on nitrox with my personal setup (with my second stage sounding like a wind tunnel in my head), but dove Sunday on a rental setup as a replacement with no problem on air during a 110' dive and a 70' dive. I haven't dived on air for quite a while and quickly remembered why nitrox is so much better
My level of anxiety was much reduced by having a nicely balanced reg, but there were still some moments where I had to secure my mind and settle down.
I've been diving for many years in some relatively harsh conditions, and not thought much about it, but lately I find myself thinking of my own mortality.
Perhaps it's just something I need to get over in my own mind and keep pushing on...