After the fact, of course you should realize that obviously this is what Hashem knows is good. Beforehand, you place your trust in Hashem that things will be good, in its true, simple, regular, meaning.
A person has to be able to disengage past experience and completely believe in Hashem. Remember and point out to yourself the times Hashem clearly answered you, and dismiss the times you were disappointed.
Emuna is like a pendulum: The stronger you push the more it comes back, and the stronger it comes back the further you can throw it. Someone with a strong Betachon sees many results, which in turn enables him to build upon his Betachon. But you can start off small.
Read Rabbeinu Bachya in his Kad Hakemach on the topic of Betachon. He says that in the Zchus of the Betachon alone you will be helped. He also adds that, of course, there are times when things fall through. Then we must focus and realize that Hashem knows best what is best.
The difference between before and after is like any strong friendship. You are positive that your friend won’t leave without you. If you think otherwise you are doubting your friendship. However, if the friend did leave without you, then an undoubting friend starts to think that obviously something urgent came up.