Negative self-esteem, "I'm a piece of crap", zero self worth

I'm trained in ACT and I appreciate is so much for many clients. But for the clients whose self esteem is so low, for many years, very entrenched, what modality do you find helpful? CBT is not what I'm looking for (I went to a CBT therapist for years and it was not helpful for this experience.) For ACT therapists, do you find it supportive when working with clients who believe "I'm a piece of SH__"?