Heart catheterization at age 9 is surely a traumatic experience, and it would be important for a mental health professional to explore his understanding of the procedure and his prognosis. Does he fear death, for example? Was the procedure successful? What is his understanding?
I recommend that you ask a psychopharmacologist, psychiatrist, or nurse practitioner, i.e., people who dispense medications and are more aware of adverse reactions.
However, I will offer some general input. First, the main and long-prevailing view of psychosis -- and manifestation of psychotic symptoms in particular, is the stress-vulnerability model. That you had a break at age 13-14 suggests you have an underlying probably genetic vulnerability to experience psychotic symptoms. The idea is that you may be doing relatively well, but if your nervous system is "stressed" which a substance such as nitrous oxide that alters one's mental state and perception of reality (i.e., such that pain is not experienced as pain), you are at higher risk for an adverse reaction in the form of psychotic symptoms.
I would be less concerned with the dental work (since nitrous oxide has a very short half-life, i.e., it only lasts a short time, than with your overall mental status and assessment of need for medication. You say you have not been able to get to a psychologist. Given your report of a break at age 13-14, all the data on early onset psychosis show that the prognosis is much, much better if you begin taking regular antipsychotic medication as soon as the symptoms manifest. So, I would very strongly encourage you to seek medication treatment as well as individual or group psychotherapy.