Teddi Mellencamp is opening up about her slow realization that she was suffering from brain tumors ... and she says she's had some of the saddest and darkest days since her diagnosis.

The reality TV star dished on the "wild" lead-up to her brain surgery on her "Two Ts in a Pod" podcast, explaining she initially dismissed her symptoms as migraines or menopause.

Teddi says she was at a horse show when she had a terrible migraine, took some of her friend's migraine meds and instantly felt worse ... to the point where she forgot where she was.

Things continued to get worse and Teddi recalled struggling to say certain words on an ensuing podcast recording.

TM says her head had been hurting for 6 months before she found out about the tumors ... but she says she ignored the symptoms because she was going through a divorce and didn't want to deal with all the bad things in her life.

Teddi gets emotional recalling those days ... and says she was just trying to move forward.

As we reported ... Teddi had 4 tumors removed from her brain earlier this month, but doctors recently discovered more tumors ... including two in her lungs.

The good news is, she's feeling well enough to return to her podcast.