The disciples have spent a lot of time with Jesus, but eventually, the disciples could not be with Jesus in person. So Jesus decided to encourage the disciples.
First, Jesus told the disciples to "let not [their] heart be troubled" but rather "believe [...] in [Jesus]" (14:1). Separation isn't always easy, and it's common to have some uneasy feelings. However, we know from Jesus that faith counters fear. We've talked about "believing in Jesus" as in "accepting Jesus as Lord and Savior in response to his gift for everlasting life" before (see John 3:16 and 5:24). However, in this context, since the disciples have already believed in Jesus for everlasting life (as implied from the already clean analogy back in John 13), "believing in Jesus" is a daily thing where we count on Jesus to counter fear and to guide our life. Even though accepting Jesus is a one-time thing, we can sometimes lose focus on Jesus and get overwhelmed by fear. Thus, Jesus encourages us to have daily faith so we don't get overwhelmed by worries. Jesus counters fear because of our security in Him.
In fact, Jesus provides various promises for the disciples. These promises can be the basis on why we can not be overwhelmed by fear.
Jesus prepares a place for the disciples (14:2). In particular, the Father's house would have mansions (14:2). In heaven, not only do we get to live, but we also get to live in a great place. Nice!
Jesus would later receive the disciples (14:3). Even though the disciples would be separated for a while, the separation would not be permanent.
After Jesus receives the disciples, the disciples can be with Jesus (14:3). In other words, they can find companionship.
Although Jesus told the disciples that they could not come with Jesus to where Jesus was going (back in John 13), the disciples can know the path of where Jesus is going (14:4).
Today, we have even more grounds to not be overwhelmed by fear. For instance, Jesus's love for us via death on the cross to counter our penalty of sin and the resurrection to counter the power of death means that we can know that we are loved and cared for by our Lord and Savior. Other Scriptures list many more promises from Jesus as well as Jesus's great love for us. Thus, we don't have to be overwhelmed by fear.