Can we tell a therapist our sins for the purpose of seeking help?

So for a specific reason like this, then yes it is permissible. In fact, it may be even needed in order to gain and help you in the best possible way. The key thing is, number one, of course the general principle: we don't expose our sins unless there's a specific need, such as in this case. And the second thing is: be very careful in terms of avoiding backbiting if it also involves other people as well.