It isn't true that Christians aren't supposed to grieve, or that it's wrong for us to experience sorrow when a loved one dies. Yes, we know they are in heaven if they knew Christ–but we still miss them terribly, and we grieve because we have lost them and they are no longer with us.
Do you remember when Jesus stood by the tomb of His friend Lazarus? The Bible tells us that He wept–in spite of the fact that He knew He shortly would bring Lazarus back from the dead (see John 11:1-45). Nor did He rebuke those who had loved Lazarus because they were weeping. The Bible says that although we know our loved one is in heaven, we still will grieve–although not "like the rest of men, who have no hope" (1 Thessalonians 4:13). This has been my own experience in recent months, following the death of my dear wife, Ruth.
Don't be surprised at what has happened to you. Immediately after a loved one dies, we may be very busy dealing with practical details. But later these fade, and holidays can be especially difficult because of the memories they stimulate.
Recovering from a loved one's death, a friend said recently, is like recovering from major surgery: It takes time, and it won't be without pain. But with God's help–and the help of others–you will recover, and ahead of you is the hope of heaven.