Please Help Save My Angel

Diagnosing fish illnesses is always difficult. Treating for internal bacteria would have been my first reaction too, though that hasn't worked for your angel.
Have you added any fish to the same tank recently? It is possible any new fish could have brought in a disease such as intestinal parasites, viral or other infections. I have recently read that protozoan infections are becoming more common - I mean internal protozoan infections rather than things like whitespot.
Unfortunately, viral infections are impossible to treat, and I don't know about protozoan. Parasitic infections can be treated is that's what the fish has, but these treatments should not be used without a definite diagnosis. Does the angel have red threads hanging from its anus when it is motionless? If it doesn't, it's not camallanus worms.


I'm sorry I can't be more help 

Ah! You say you've had the affected one for 2 years and have just added a couple more. Angel fish are very territorial and it is very difficult to add new ones to a tank where there is an established angel. I would have expected the old one to have attacked the new ones but perhaps the new ones won't stand for this and have attacked the old one and that one is getting very stressed
Do you have just these three angels or are there more that have been in the tank longer than the recent couple? It's just that three angels is a bad number. The recommended angel stocking is one or a mated pair (that is a pair that have chosen each other not just 2 fish) or half a dozen, if the tank space allows.