If you can do nothing more, at least continue saying 'I-I' to yourself mentally all the time, as advised in Who am I? ., whatever work you may be doing and whether you are sitting, standing or walking . 'I' is the name of God.It is the first and greatest of all mantras. Even OM is second to it.
'Who am I?' is not a mantra. It means that you must find out where in you arises the I-thought which is the source of all other thoughts. But if you find this vichara marga too hard for you, you can go on repeating 'I,I' and that will lead you to the same goal. There is no harm in using 'I' as a mantra. It is the first name of God.
God is everywhere, but it is difficult to conceive Him in that aspect, so the books have said, 'God is everywhere. He is also within you. You are Brahman.' So remind yourself: 'I am Brahman'. The repetition of 'I' will eventually lead you to realise 'I am Brahman'.