Sadly, OP, you were a little naive. This is the equivalent of falling for a scam - I’ve learned over the years that when people couch business in terms of ‘you must decide today or the offer expires at midnight’ it is ALWAYS the case that you say, ‘then, I’ll have to pass this time’ and you show them the door.
I’ve become more confident doing it as I’ve got older (I am guessing you may not be in your 50s with kids about to/who have fled the nest so may not have as many grey hairs as me) but you never buy the ‘if you don’t contract today, I won’t be able to fit you in’ line.
I am guessing this error may have cost you somewhere to the tune of £5k+ and this may have been a significant and hard earned [and saved] lump sum for your husband. The psychological impact of ‘being scammed’ - as in effect you were - can be deep reaching and long lasting, hence your husband’s apparent downward spin into what may be depression.
I think from the tone of your replies you are focused on the fact that he is being grumpy and not forgiven you - but you don’t seem to realise the true impact of what you have done? Unless you can actually acknowledge the depth of the impact of your misjudgment, you can’t truly apologise, can you? And apologies mean nothing without subsequent changes in behaviour - otherwise, they’re just words.
I do think, given that it seems to have triggered a period of [mental] illness for your DH, that you need to sit with him, acknowledge how bad an error this was and ask him what you can do to rectify it. It may mean saving up or taking out a loan to repay him, it may be counselling to explore how you can learn to listen to each other better - but, ultimately, you need to ask him to explain what he wants from you going forward and you need to act on it.