Doubt is like a disease.
Once you have it, you have a limited cure for it.
You can doubt yourself and your capabilities.
And die in pain and misery for everything you failed to achieve.
If you doubt your partner, your relationship is a shrinking ship.
They will do whatever to persuade and bring trust, but it will fail.
Two of you will end up living like aliens under the same roof.
Doubts are a manifestation of our brain. It gets triggered because of our inferiority complex.