I'm upset with my wife for going back on paying our down payment
Have you discussed the matter with her?
Yes and she says it's enough for her to pay half of the house payments
If she said she would do it she should. You guys are building a family together and you need to have trust.
Yes. She's paying half now but she wants to change that half to third. She's being grossly irresponsible here.
You are trying to star a family with her. You need to trust her with finances, and she's failing in that regard. Talk to her and a financial advisor immediately.