Why so difficult to trust?

Experience.

When you have been deceived and hurt, it is natural that you develop trust issues.
You just need to learn from the experience(s) to identify the signs and red flags earlier, so you can make educated decisions towards staying or walking out.
In any case, we just have to have a leave of faith!

Once bitten twice shy.

agree it's part of life life is bit they will gain strength from it.