The joy is in that you've found your King, your Lord, and your Savior.
Like, you know you have a purpose, and that it's not all pointless, that even when you suffer and struggle there is a point to it all that is going to be for your good.
This might seem silly but something that's kind of stood out to me recently. Most, if not all people have a deeply held desire for a hero/savior, I think it's God given. That's why we like movies and stories with heroes in them, that's why "hero worship" is a thing, that's that desire twisted and misused in the world. We have a desire for Christ as our Hero and to worship Him.
But anyway, so I was watching just some youtube videos.. reactions to certain scenes in movies and tv shows, for instance, the moment in "Avengers Endgame" where Captain America is able to pick up Thor's Hammer, or in the Mandalorian TV show where they have Luke Skywalker come and rescue the protagonists. I mean the setup is the protagonists are in great danger, the villains look like they're going to win .. then.. the camera shifts over to the Hammer starting to move (in the Avengers).. and I remember the theater got really excited. There's anticipation, this feeling that this desire they have, for a hero that they love to have his time to shine is about to be fulfilled, or in the star wars show, when the X wing shows up to the ship. There was that anticipation, the audience knows who they want it to be. As the hammer snapped into Cap's hand I heard someone behind me just say "yes!" This was something they hoped for and when they saw the signs that it was going to happen there was fulfillment and joy in that, and as the scenes go on in those videos the members of the audiences are reacting with joy and sometimes with tears in their eyes.
Understand that Jesus is the true fulfillment of that desire for a hero, a savior, saving us not only from our sin, but someday from the darkest and most dangerous times this world will ever face, and He's coming back to save you and defeat all our enemies, sin, death, the wicked of this world, and Satan himself. As a Christian you are seeing the hammer begin to wiggle metaphorically speaking, you're seeing that X wing make its approach right now, and there's that anticipation that your Savior is coming, a Hero that you love is about to have His time to shine, when He returns. That is part of the joy in being saved, that we know our Hero is coming back that we will have true fulfillment of that deeply held desire for a savior.
The peace comes from knowing that He has done what is impossible for you to do (win the battle against sin by yourself), so that you can be reconciled to God.