Note 2 at Joh 14:1: The understood subject of this sentence is "you." "You" let not your heart be troubled. You are the one who has control of your heart. Other people and circumstances cannot trouble you unless you allow them to divert your attention from the Lord and His Word (Isa 26:3, see note 39 at Joh 14:27).

Note 3 at Joh 14:1: Jesus had just told the disciples not to let their hearts be troubled, and here He told them how to do it. Believe! Faith in God is the victory that overcomes the world and all its troubles (1Jo 5:4).

Note 4 at Joh 14:1: The disciples believed in Jesus enough to be totally devastated when He died but not enough to believe His prophecies concerning His resurrection (see note 1 at Mt 16:21). Jesus made it very clear in this same teaching (Joh 16:1) that He was saying these things so that His disciples would not be offended, but His words didn't profit them, because they didn't mix them with faith (Heb 4:2).

They had enough faith to be dissatisfied with failure but not enough to have victory. It's better to go all the way in believing God than just part way.