chris,
I'm concerned that, for whatever reason(s), you didn't have a solicitor present during your interview. The decision must have been your own, because you should have been advised by the custody sergeant prior to the interview that you were entitled to one free of charge?
Well, it's too late now, but just for future reference, if you're unlucky enough to be ever nicked again for anything, I'd strongly advise you to have a solicitor present, for, with no disrespect, s/he can advise you on the finer points of the law in case you unwittingly drop yourself in it.
I have no doubt that at your first Court appearance on Tuesday (Magistrates Court?), you will be advised to get a solicitor to represent you before they proceed with the case. I don't know if you qualify for legal aid, but all the better if you do, for then you will have one provided at taxpayers' expense. Otherwise you could end up with a hefty bill.
If possible, you could contact/go to your local Citizens Advice Bureau tomorrow where you can be given free legal advice. They won't take it any further than that but should at least be able to give you an idea of where you stand. I'm assuming there that you haven't got a solicitor?
It has also been known for someone to withdraw a complaint altogether which would mean in all probability that no further action would be taken in the matter. I mention that because you say she is/was your girlfriend, so it goes to follow that she has/had feelings for you, and it is not unknown for people under those circumstances to "forgive and forget". However, that's the brightest scenario - so don't bank on it.
Be very, very wary of trying to represent yourself. The Court will expect you to have proper legal representation and that is something in your best interests too, so think carefully - and admit to nothing which may incriminate you. The onus is on the prosecution to prove the case