For a small moment have I forsaken thee, but with great mercies will I gather thee.
In a little wrath I hid my face from thee for a moment, but with everlasting kindness will I have mercy on thee, saith the Lord thy Redeemer.
For this, the waters of Noah unto me, for as I have sworn that the waters of Noah should no more go over the earth, so have I sworn that I would not be wroth with thee.
For the mountains shall depart and the hills shall be removed, but my kindness shall not depart from thee, neither shall the covenant of my peace be removed, saith the Lord that hath mercy on thee.
I think its easiest to understand these things when we remember who is promising them. It is the Lord Jesus Christ promising His people that He will not forget them. He gave His life for us. Remembering that, and understanding that, will help us not to doubt.