As the bowler runs in, lift the bat backwards in the direction of the stumps. Keep raising the bat until you feel glad. If you start to feel depressed and resentful, your bat is too high.
Ensure your shoulder rolls counter-topwise at the moment the bat reaches its highest point and adopt a relaxed tension and detached concentration. As the ball is delivered, you should be swift yet static overall.
A good, textbook backlift feels comfortable but only within the bounds of discomfort. Unqualified comfort indicates a major technical flaw, so if you find yourself in this position, find written instructions as to how to execute a textbook backlift and try and identify where you are going wrong.
The problem is likely to be that you are lifting the bat in a manner outside the bounds of discomfort, so pay particularly close attention to any advice surrounding this aspect.