"Blessed are those who mourn for they will be comforted." Matthew 5:4
The prerequisite to receiving God's comfort, is that we mourn. It is not wrong to grieve and we need to give ourselves permission to feel the pain of our losses. Yet we don't mourn "like the rest of men, who have no hope" (1 Thessalonians 4:13). Our mourning is different mainly because of our hope in Jesus who has overcome the power of death. But also because Jesus is our High Priest who has shared our humanity and is able to sympathize with us (Hebrews 4:15). At Lazarus graveside Jesus entered into the pain and wept. He showed us that tears and grief are part of the process of coming to terms with our losses.
Even when we experience smaller losses we still need to acknowledge the pain and mourn. The "stiff-upper lip mentality" isn't God's idea. I once heard a worship leader makes this comment, "Let the hurts of a life time flow into His nail scarred hands." Once we have felt the pain we are then free to let it go. Even then it's a process and not an instant pain-killer.
Being a Christian doesn't guarantee us a life without tragedy but being a Christian means we have access to God's resources. He promises us His comfort when we mourn but if we don't mourn we can't receive God's comfort. God encourages us to come to His "throne of grace with confidence so that we may receive mercy and find grace to help us in our time of need." (Hebrews 4:16)