Its ok, as its not really about meaning that you should be scared of relaxing, its just that this particular method doesn't work for you. If you want the truth, it doesn't for me either. I use breathing exercises without even knowing I do it these days, but not for relaxation, instead the other type which ensure we get full lung capacities all the time, and not the short shallow breathe we tend to take when stressed. I tried all these breathing relaxation things, and none worked for me, as I just generally couldn't concentrate enough upon them.
Don't worry Piglet, all it means is that that exercise isn't the right one for you. There are plenty of relaxation methods. Some people do Yoga, some Tai-Chee, some go to the acupunturist, some get massages, some walk, some go running, so go to the gym... etc etc... you get the point.
There is no definitive answer, but more its trial and error. The problem generally with PTSD, is getting people to actually get past their thoughts and try things. I have mentioned this in regard to myself and acupuncture, as I was hell bent on never getting it, finally said I would give it a try, and well, I have to eat my words. Admitting I was wrong was harder than going to get it... so pride can bite us on the bum at times with these things.
The more you try, the better judgement you will have at the end of the day to what works and doesn't work for you.