If you turn up unrepresented and the magistrates are considering custody they may offer you an interview with the duty solicitor (which, of course, you can decline). Their sentencing guideline “starting point” for Theft “In Breach of Trust” (which this probably is, though you do not give the details) is a custodial sentence and your offence is aggravated by the reasonably large sum involved and the length of time over which it was committed.
Whatever happens and whether you are represented or not the Bench will almost certainly ask for probation reports on you before sentencing (and you must co-operate in their preparation). The probation service will look into all the circumstances surrounding the offence and your own circumstances, including the health matters you have outlined. The final decision is not a matter for you or your lawyer (if you still have one) but for the magistrates.