Sentencing in assault cases is a complex matter. The judge has to consider both 'harm' and 'culpability', using those factors to determine which of three categories the offence falls into.
We've not got all of the information that the judge will have regarding the 'nitty gritty' of the offence but I'd hazard a guess that it might fall into 'Category 2'. If so, the judge will then have a 'starting point' of 6 years' imprisonment to work from. That might get pushed up to 7 years because of your partner's previous record of violence, and the other matters you've referred to, but then get cut back by a third, to 56 months, for an early guilty plea.
Only half of a nominal sentence is spent 'banged up', so my best guess is that your partner might be 'out of circulation' for a bit under 2½ years. (The other offences can probably be ignored because the sentences for them are likely to run concurrently with the one for GBH)
To read exactly what the judge will have to refer to, see pages 3 to 6 here:
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
You can't withdraw a statement. The statement you originally gave can still be read out in court, with you being asked why you now wish to change it. Judges are specifically advised to ensure that there's no possibility of a victim of a domestic assault being coerced into changing their statement, so they're encouraged to take a victim's
original statement as being true unless there's very clear evidence that it's not.
Further, if a court believes that an offence has definitely occurred but the victim refuses to provide the relevant evidence, it's even possible that the victim could be sent to prison. (See here for an example of when that's happened:
https://www.bbc.co.uk/news/uk-wales-mid-wales-11707903 ).