Question

I realised later the mistake had been mine begin with only

Answer 1

Only later did I realized that the mistakes had been mine.

Answer 2

The sentence can be started with 'only' as follows.

'Only later did I realize that it had been my mistake'.

As per the Cambridge English Dictionary, The used of 'only' can be done as an adverb in various positions, as per its focus.

If a subject is itself the focus, it can be put in the front position.

As an example, it can be said, 'Only one out of four people knew the address' or 'only a person could have addressed the problem', etc.