Greetings 100th Monkey, welcome to the nexus.

I have heard many reports of SSRIs or NSSRIs (because they interact with many of the same receptors) having a diminishing effect on psychedelic trips. I suspect this would be because the increased concentration of serotonin and noradrenaline in the synapse out-competes the psychoactive compounds at those specific receptor subtypes. This would explain why you were able to smoke so much with minimal effect.

I suggest that if you are really interested in the healing power and pure consciousness shift (enlightenment) of the psychoactive experience DMT has to offer, you need to fully stop taking the SSRI for 2-4 weeks before you try DMT again. 

Yes your body is physiologically dependent on the SSRI, but the withdrawal syndrome will pass within a week or so. (quite nasty, but not impossible to endure)

Once you do quit, you do not necessarily need to inhale DMT in order for it to work. Eg. Pharma/Ayahuasca (but if taking a MAOI, it is definitely important that you are not on an SSRI)

Good Luck!