Just to be clear and put your mind at ease a bit, Tramadol is not a serotonin releasing agent (which reverses the action of serotonin transporters that normally vacuum up serotonin from the synaptic cleft, reversal causing serotonin to spill out into the cleft) but rather it just inhibits the reuptake transporters from vacuuming it up.
But regarding measurements after chronic SSRI use (serotonin releasing agent use), results aren't very clear, and the genetic changes that lead to SSRI efficacy are still being unraveled. But it's clear that people with different genetics respond better than others. Some studies have shown that even chronic MDMA abusers SERT levels return to normal given enough time. But in general decreased SERT expression is probably a compensation for lower serotonin levels acutely after MDMA.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4207076/?report=classic -
"Experiments by Launay et al. [14], performed on neuronal cultures from raphe nuclei (and serotonergic neurons induced from the 1C11 cell line) demonstrated that 5-HT2B receptor-PKC coupling promotes phosphorylations of SERT that control SERT activity. More specifically, it was shown that 5-HT2B receptor stimulation in the absence of added serotonin approximately doubled phosphorylation of SERT serotonin transport, a result very different from the generally accepted ability of fluoxetine to reduce SERT activity [14]. This pronounced difference may be explained by substantial differences between the immature and the mature brain [15, 16]."
https://www.ncbi.nlm.nih.gov/m/pubmed/10575045/ -
"he serotonin content of the hippocampus was unaffected by paroxetine or sertraline treatment, ruling out neurotoxicity as a possible explanation for the SSRI-induced decrease in SERT binding and alteration in 5-HT clearance. Levels of mRNA for the SERT in the raphe nucleus were also unaltered by chronic paroxetine treatment. Based on these results, it appears that the SERT is downregulated by chronic administration of SSRIs but not other types of antidepressants; furthermore, the downregulation is not caused by decreases in SERT gene expression."
Hope this was helpful, feel free to ask any questions. Ultimately I wouldn't worry about taking SSRIs if you feel you need to, and I wouldn't worry about having taken Tramadol in the past. Treating stressful conditions is probably more important than the long term effects of SSRIs. And in your case the real harm could come from simply worrying about the long term effects. You have a lot on your plate at 18! But I promise things will smooth out with time. Try not to think too much, although if you like learning about neuroscience-ish things because it occupies your mind in a good way then that's okay in my opinion, it helped me anyways.