Teddi Mellencamp's on the road to recovery ... 'cause she says her tumors have shrunk significantly -- and, her doctors think she's just a few weeks away from beating cancer.

The reality star took to social media Wednesday to share the good news with her fans ... announcing it looks like her treatment is working because the tumors on her brain are much smaller than they were before.

Teddi says she has two more sessions of immunotherapy, and then she hopes her treatment will be wrapped up and she'll be completely cancer-free, as her doctors believe she might be very soon.

Teddi says she's trying to keep a positive outlook, just like her doctors ... who she adds were also making a couple references to her dad, John Mellencamp, at the office.

As we told you ... Teddi announced a medical issue back in February, revealing she dealt with blinding headaches before going in for the appointment -- and, she found out she had a number of tumors on her brain.

She underwent surgery to remove some and started radiation to get rid of the rest ... when doctors found even more tumors on her brain earlier this month.

It seemed grim for Teddi ... especially after she admitted her dad was already planning her burial -- but, it sounds like she's far from the grave now.