When you contradict her by telling her that she is not behind in life, you are showing her that she is alone, that you, and people generally, cannot understand and accept what she is thinking and/or feeling.

As an alternative, you might want to try "active listening".

In active listening, you "reflect back" what the person thinks and/or feels, without judging it. So, for example, when she says that she is behind in life, you might say You sense that you will never catch up. Or You feel inadequate and hopeless right now. Or You see your cousin as doing so much better than you right now, and it hurts to see that.

These statements do not agree or disagree, they simply show that you understand her thoughts, and/or you understand her feelings, and you don't judge them. You simply are with her in her distress. She is not alone.

If you don't quite get her thoughts or feelings right, she has the opportunity to correct you. If she does correct you, then you just reflect back the correction--again, to show her that you understand and accept without agreeing, disagreeing, or judging. You simply are with her in her distress. She is not alone.