My future mother-in-law said that my girlfriend and I were not a good match. Of course, neither of us believes this, but she can't say it. What should I do?

Then mobilize other relatives and friends who support you two to persuade your future mother-in-law.

It's better to add some examples, such as who doesn't fit in with whom, but still live happily together.

or

Ask her what to do to solve the problem of fate. If there is a problem, there is always a solution. Although superstitious, there will always be one.

Finally, I wish you both an early and happy marriage!