I cannot speak to why your marriage was nullified, but this statement caught my attention:
He hasn't given me my mehr, as was
discussed before the marriage and said we were not going to do, was
In my opinion this is not right, because mehr is not optional
And give the women [upon marriage] their [bridal] gifts graciously.
But if they give up willingly to you anything of it, then take it in
satisfaction and ease.
It must be given and is a right of the woman; now if the woman chooses in her good will to give you (the husband) anything from it, you (the husband) should accept it without hesitation.
In marriage contracts it is clearly written the amount of mehr, and how is it disbursed (for example is it in the form of cash, property in the wife's name, jewelry, etc.) and it is typically given at the time of signing of the contract. It is also clearly announced when the contract is being signed and is witnessed.
In some cases it it deferred - by that I mean, not paid at the time of the contract signature, but this is again clearly mentioned in the contract the amount and type; and it is considered a debt on the husband till paid.
Mehr is not something that is given "at the time of divorce", but it is a condition of marriage.
He left me and our home after three weeks. I think it was a way for
him to end a marriage which he nor his family wanted from day one.
This is more of a personal issue with you and your husband and should be taken up between you and the elders in your families as is the custom in Islam.