Once it's done cooking, and has cooled, if it's still tough, it's only hope for tendering up, in my
experience, is changing up how it's served. So, very thinly sliced, across the grain, for sandwiches or frying, or cut in small chunks, for hash, soup, quiche, or casseroles, are ways we've used it.
My preferred method of cooking it is overnight, in the slow cooker. I've honestly never had it come out tough, that way. I've had people going nuts before breakfast, wanting it, though, so buyer beware, lol.