Hey buddy, welcome to Bluelight!
First and foremost, as you've probably begun to realise and will be told by others time over - your usage is obscene to say the least. The fact you've somehow not be burned at all is somewhat of a miracle, let alone severely. Anyway...serotonin syndrome is an acute condition (as opposed to chronic) whereby it develops rapidly and severely in most cases. It's not something you develop over time, but something relatively immediate as a result of drugs or particular combination of drugs. My understanding is that for you to manage serotonin syndrome from MDMA alone, you'd need to ingest a ludicrous amount in a very short time. The more likely cause of serotonin syndrome will be combining MDMA with something like an MAOI (Monoamine Oxidase Inhibitor - something that inhibits the action of the monoamine oxidase enzyme which is responsible for metabolising a great many molecules inclusive of serotonin. The combination will cause a release of serotonin, but inhibit the metabolic process that would normally remove the excessive serotonin, causing rapid build up to dangerous levels.
I believe you can also cause serotonin syndrome from combining a serotonin releaser like MDMA with an SSRI (Selective Serotonin Reuptake Inhibitor), presuming you've not been on the SSRI for some time. I'm not really sure about this scenario, so don't take my word

really, don't take my word on anything, but instead make your own informed conclusions on risks and preferably err on the side of caution - your life isn't worth the gamble of uncertainty.
So besides that, you REALLY need to cut back on your usage before the side effects your mates talk about catch up to you in a bad way, and they will. Trust me, as much as some would like to insist or believe they're different, you're not and we're ultimately just rolling a a dice with a different number of sides. Statistics insist your number will come up.
Someone else will chime in with some more info, likely berating you for your abuse of MDMA, but alas I must continue my mission to get the hell out of work and go home. Our advice may come across a bit harsh, but really it's harsh love. Nobody here likes to see another happy roller burn themselves out
