You're becoming resistant to it. It can be hard to OD on something you take daily (I advise you stop) but some of the more powerful seratonin drugs, like the tricyclic amitriptyline can shut down any body with enough, because attempting to digest it makes it more powerful for three cycles.
Serotonin poisoning can shut down/disorder a large array of ionotropic glutamate receptors, including adrenaline, so the cause of death will likely be heart failure.
I tried 4 boxes of cough pills, which included over 200mg antihistamine (200mg kills 50% of the population) woke up in the happy hotel with the walls melting. I decided my experience was so profound and amazing, that I want to keep living.
There is always the chance of waking up with horrible injuries, and maybe even becoming too incapacitated to finish the job. I suspect more surefire methods would involve a mixture of benzos, pain killers and alcohol, death via sleep apnea. We aren't made to kill ourselves and shouldn't. If you stick through life enough things will get better.