L-methylfolate

I'm not big on supplements, but this one has shown promising results combined with an antidepressant and can facilitate mood regulation. Might save you from a dosage increase of a med or the need for the addition of an adjunct medication to your current regimen.
Please run it by your practitioner first, because as with many supplements, it's contraindicated for use with other medications. It's reasonably priced at about $13 for a 3mos supply.

I've just been reading up a bit on it as this is the first I've heard of using this approach. Can fully understand the rationale behind this as an adjunctive treatment with some studies showing positive results for TRD (SSRI).
Definitely need to run it by the practitioner first as it can interact with quite a few meds. Also, depending on how well it can enhance antidepressant effect, that might actually mean a potential lowering of dose of antidepressant. Which of course would be a clinical decision.