OP you need to decide (and quickly) do you want to retrieve the situation, and retain your job.
They are giving you the opportunity to give your side of the situation, and recognising your right to be accompanied. They sound like a reasonable employer.
You need to be mindful that with your 2 year anniversary still 2 months away, they could come down heavy on this situation citing your lack of attention on the job, when call centres are generally very tight on their stats, service level agreements etc, which you may have adversely affected.
If you are resolved to keep your role (judging by your comment that you and your DH are reliant on your income as well as his), I would make some notes of the criticisms they have levelled against you, with credible responses as to what happened, and how you intend to tighten things up significantly. You also need to draw their attention to your MH condition and if you are certain that it may have contributed in some way to your lower parr performance, then say so, but try not to use it as an excuse, just a way of explaining how things deteriorated.
Ultimately they see you as a problem employee, and are highly likely to use it to part company. Only you can decide if you want to fight to keep your role. Currently I'm afraid your posts come across as muddled and disorganised, so try to strip away all the confusing detail and summarise some key facts to talk through with your union Rep to prepare for the meeting.
I wish you all the very best.