Do you have a separation agreement or court order that dictates how much and when?
If you do, then you need either her WRITTEN agreement, or you have to go through the lawyer.
If neither of those is an option, then what I would suggest to you is to SAVE 375 on the 15th and pretend that it's for your support. Then you'll have the extra on the 1st. Slide it into a savings account until then if you need to.
Yes, it means for the first month, you'll have to tighten your belt a hair.
A budget is relative. If you KNOW you have a big lump sump due on the first, then you take a little bit on the 15th and offset the upcoming big payment with it. Is your bank maybe willing to play with the mortgage dates instead?
IF you are in Ontario, then you can try the FRO, however they are a nightmare to deal with (or so I hear) and they'll enforce the order.