Boy, you must be very frustrated. I am so sorry you had this experience.
The reason your porcelain veneer came off after five months is most likely due to 1) your bite or occlusion 2) you bit down on something hard that dislodged the veneer 3) when the dentist bonded the veneer onto the tooth there was some contamination that weakened the bond between the veneer and underlying tooth.
I recommend returning to your dentist immediately. If you have a full, intact porcelain veneer, then your cosmetic dentist can clean out all the old cement and rebond it back onto the tooth. If you broke the porcelain veneer, then the veneer would most likely have to be remade.
With regards to why your gums are still inflamed after five months, most likely one of two things have happened. 1) the edge of the veneer at or below the gum line is not smooth, collecting plaque and irritating the gums. 2) you may not be brushing and flossing around your veneers adequately.
Your cosmetic dentist can check around the edge of the veneer to see if it is smooth, and if there is any residual cement material that might be irritating your gums. Usually this can be corrected without remaking the veneers.
I understand your anxiety that your veneers might fall off again. If you find that this repeatedly occurs you might consider having them all redone. If after five months you are still experiencing sensitivity that might indicate that your underlying tooth is not healthy, that any decay and drilling might have overly irritated the nerve in the tooth, or possibly the veneer itself is leaking, allowing microscopic basteria to crawl in underneath the veneer and is irritating the tooth.
First I would recommend returning to the dentist who placed your veneers. If you are not satisfied, then seek the opinion of another cosmetic dentist.