If I were chronically sad in the evenings, I would actually bring it up with the doctor. I wouldn't make a special appointment for it necessarily unless its progressing towards suicidal thoughts or something, but during a regular visit I would bring it up. I'm no stranger to mood disorders, and frankly, I wish I had approached a professional sooner about it. Would have saved a lot of grief not just for me but for family members who also suffered through my episodes.
For something that seems too minor for a doctor visit, I would suggest speaking to a pharmacist. They are readily available without appointment or long waits, and have an encyclopedic knowledge of all sorts of over-the-counter remedies that are backed up by an advanced degree in medicine. Definitely an underutilized resource. Of course they may well suggest you talk to a doctor, but in my experience at least, they give thoughtful consideration to what you have to say and make such a recommendation not out of some lazy buck-passing but rather a genuine concern for your condition. So you would do well to heed their advice, whatever it might be.