Imagine that you are scheduled to undergo open heart surgery tomorrow. You've met with your surgeon, and you have complete confidence in your surgeon's skill and personal care for your body. Although you're naturally anxious about such major surgery, you anticipate a successful outcome. There will be a lot of pain, but your surgeon has told you what pain to expect, how long it will last, and has prescribed medication to make the pain tolerable. Your trust in your surgeon has put your heart at rest.
Jesus is the Divine Physician (Mk 2:17). He has decided that the best way to bring us to full health is to give us a new heart (see Ez 36:26). In the process of this heart transplant operation, Jesus may, in His surpassing skill, care, and wisdom, find it necessary to expose us to pain for a time (see Lk 2:35). But Jesus didn't promise to eliminate all pain; instead, He promised that by having faith in Him, we can keep our hearts from being troubled (Jn 14:1).
Out of love for us, Doctor Jesus has not told us everything about our operation and recovery because we are not yet ready to bear it (Jn 16:12). We don't know what kind of pain we'll experience or how long it will last. The pain may come physically, mentally, spiritually, in relationships, etc.
So, it all comes down to believing in the Doctor. Doctor Jesus, the Creator of your body, knows more anatomy and has far greater personal care for you than does the world's finest physician. Jesus tells us: "Have faith in Me" (Jn 14:1). "Dismiss all anxiety from your mind. Then God's own peace, which surpasses all understanding, will stand guard over your hearts" (Phil 4:6-7).