Then your requirements are in conflict with security. You cannot trust the network to do your encryption for you, and you can't trust a a computer you don't own

Then your requirements are in conflict with security. You cannot trust the network to do your encryption for you, and you can't trust a a computer you don't own to handle your private key. You may want to reconsider either dedicating some sort of portable device to be your email that you carry with you, a multi-account system the separates private email from the the email you can access remotely, or resigning yourself and those communicate with to sending using only postcards (non-end-to-end-encrypted email).