Hey Solipsis!
I would first like to thank you for taking the time to construct such an informative and meaningful answer to my question, seeing as I am new to Bluelight and psychedelics. I hope that I can do the same for you, since you took a definitive interest in my problem.
I will admit to you that I have inappropriately catagorized my experience as being a flashback, and I appreciate your explanation which has helped me better understand the literal meaning of flashbacks. Like I said, I'm relatively new to these drugs and I have little to no understanding of their lasting effects, so I am grateful for the guidance.
I would like to preface this discussion by saying that I smoked a moderate amount cannabis on two separate days in the two week period between when I ingested the mushrooms and began taking medication again. This cannabis use, on both occasions, did NOT produce any hallucinogenic effects similar to those experienced during my trip. I bring this up because you used cannabis as an example when describing drug use triggering the effects of another (in this case, psychedelics). I wonder if this effect has a name, and also why the cannabis did not cause me to experience these psychedelic effects while the SSRI did. I'm assuming it has to do with the fact that psychedelics and SSRIs affect the same neurotransmitter/bonding sites in the brain, but what do I know, I'm a Greenlighter

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Anyway, what confuses me is the fact that I experienced no HPPD-like effects for the full two weeks up until I began taking an SSRI again, regardless of whether I was sober or using cannabis. This is why I'm not sure if it actually is HPPD or some type of short-lived version of it induced by SSRI medication (I am hopeful for the latter).
You suggested that the deregulation caused by the psychedelics may be interacting with the SSRI, causing me to experience these odd effects. In your opinion, do you think that these effects are permanent or do you think they will decrease over time and/or with cessation of use of my SSRI medication? I know you suggested that I work my way up the dosage ladder again, but I am concerned for my perception in the permanent future, which is why I have decided to cease using SSRIs for good and look for an alternative to solving my anxiety problems. It may be therapy or a natural remedy, who knows. But after this experience, I'm not sure if I will ever be able to use this type of medication again. Considering I was able to go off of them the first time without any significant withdrawl symptoms, I am not worried since I have only been on them for roughly a week this time around. I can only hope that I have not developed permanent or semi-permanent HPPD from this interaction, but I guess only time will tell for me.
In response to your final question, I have never felt specifically "unstable" but I have been experiencing rather significant depersonalization/derealization since the first day I noticed my perception was altered (roughly four days ago). Therefore, I have been feeling "out of it" during this fiasco.
I look forward to your response and thank you for your assistance!