I'm not trustable.

[Silverobama]

Frank and Kevin have known me for a few years. They are university teachers of translation and interpretation. In the past few years, I sometimes referred tutoring work to them while they helped me in return. They are good friends and very close to each other because they've known each other for many years. Frank once said maybe one day we could work together, teach students English and earn money altogether. I know in private he usually does this with Kevin but not me, but they often say that my English is good to teach some students. I think they don't believe me so they decide not to include me in their teaching plans.

I wrote the sentence "I'm not trustable to Frank and Kevin".

Is my sentence natural? If not, what can I say?

 

[stanislaw.masny]

A native speaker can comment on this. I would formulate it this way:
"I'm not worthy of Frank and Kevin's trust."

I'm waiting for a professional answer.

 

[tedmc]

Frank and Kevin don't find me trustworthy enough to work with them.

 

[emsr2d2]

Frank and Kevin have known me for a few years. They are university teachers of teach translation and interpretation interpreting at a university. In the past few years, I have sometimes referred tutoring work to them while and they have helped me in return. They are good friends and very close to each other very close friends because they've known each other for many years. Frank once said maybe one day we the three of us could do paid work together, teaching students English. and earn money altogether. I know in private he usually already does this privately with Kevin but not me, but they often say that my English is good enough to teach some students.

Note my corrections.

I think they don't believe me so they decide not to include me in their teaching plans.

This makes no sense. What don't they believe you about? You haven't said anything!
In the preceding sentence of your text, "... they often say" means "Frank and Kevin often say". If they say they think you're good enough, it doesn't make sense to suggest they don't believe that you are.

I wrote the sentence "I'm not trustable to Frank and Kevin".

No. For a start, the word is "trustworthy", not "trustable". The word order is unnatural. Tedmc has already given you a good wording.

Is my sentence natural?

No. See above.

If not, what can I say?

See tedmc's suggestion.