The 'Tiger King' recently sent us a letter, saying he just wasn't getting enough support from Seth to help him get sprung from prison, while also claiming Seth could no longer take the public pressure of dating such a famous figure.

In the one-page letter, Joe gave Seth props for speaking with him by phone for the last 2 years and helping him "mentally fight cancer." Joe also wished Seth and his "amazing" mom all the best in the future.

But, Joe had one more surprise, teasing that he has a new beau ... "Someone older, much more mature and supportive will be a huge part of my life soon."

As you know ... Joe has been a dude magnet in federal prison ever since he was convicted in 2020 for a murder-for-hire scheme and sentenced to 22 years.

After his split from Dillon Passage in 2021, Joe held an online 'Bachelor King' competition, accepting applications to be his new boyfriend. Seth applied and became the lucky winner, but the two broke up and then got back together. In between, Joe briefly dated fellow inmate John Graham, who cut off the relationship after he was released from prison.

Then there's Joe's very public battle with prostate cancer, which has complicated matters. As we reported in February ... Joe didn't think he had much time left on this Earth so he signed a will, leaving everything he had to Seth.