You can't really validate their feelings, you have no say. You validate your partner. You let your partner know you care.
Ask you partner to 'tell you more' when they are talking about their feelings. Don't give your opinion. Ask them when you feel upset about this, how does it affect you. Get the impact from your spouse. If you are a man....don't try to fix this. Don't tell your partner how to feel better. Just really listen without judging whether it is good or logical or beneficial.
It is hard to begin with, and it takes practice. It is an incredible skill....but beneficial not only with your partner, but friends and coworkers. I recommend you practice there as well. Warning: More people will come to you with their feelings.
