You can still be mindful when you’re doing metta, and it is similar to watching pain. I call it strategy. When you can’t really face it, you make yourself more comfortable to face it another day.
When we find ourselves overwhelmed by a difficult emotion that we can’t handle, for some people, they can make that a metta practice. You could also do reflections of the Buddha or any of the other Brahmaviharas. I had relied on a neutral object – my breath, music and so on.
The goal in mind should be that this practice is to strengthen the mind so that it can come back and face the real problem another day. Keeping that in mind then we won’t just be spiritual bypassing but using it as a tool, a stepping stone, to come back to the problem so that we can really deal with it more effectively.