Heaven, paradise etc.

[Edwardo Piet]

isn't 'you' defined as what you currently are and exist as right now.[?]

No. Me in the past and me in the future is, by definition, me. It's me in the past and future instead of me in the present.

'You' are the summation of every moment leading up to this one;

You contradict yourself. Now you are implying that I am myself in the past right up until myself at the present. But you don't mention how, of course, the future me is me in the future.

A theoretical entity that was different from you in any way would no long be 'you'

Me at different times is me. It's me at different times.

It would be an entity very similar to 'you', but not quite the exact same 'you'.

No, it's me. It's me at a different time.

I am being myself now, I was being myself before (whether that's me before I was born or after) and I will be myself later (whether that's a living me or a dead me).

I was, I am and I will be, myself.