First of all it's completely understandable for you to feel frustrated and stressed in a situation like this. You really don't have the power to fix other people's problems. Have you ever been to a therapist? The one constant among good therapists is that they don't give you advice. They don't tell you what to do. And they don't try to fix the problems in your life. But they do end up helping you. So what do they know that the rest of us don't? They know that the best way to help someone is to listen supportively, to validate their feelings, and to give them a chance to think about their life while leaning on the emotional support of someone who cares about them. There's more but that's a good start. And if you can do that you really are helping a lot more than you think.

Take as an example, a conversation I had with my wife today (remembered as well as I can). She was in a really bad mood and wasn't talking with me.
I asked her "what's got you feeling down today?"
She angrily said she didn't want to talk with me about it because she knew what I would say and she didn't want to hear it.
(That, by the way, is a pretty clear hint that someone is not in present time emotionally.)
I said "so you're saying I'm never supportive of you when you're upset?"
She said "I'm feeling overworked and stressed. There's always too much to do. And I know you are just going to say you also have too much to do."
I said "I did say 'I feel the same way' once when you said you could use a day off. But that doesn't mean I can't appreciate that you're overworked. I understand. There's a lot going on right now and it's hard to keep up."
She continued talking about how she felt. I continued validating her feelings. And she cheered up and her mood improved.

My point is, "just being there" is code for listening supportively, validating someone's feelings, and helping them regain the balance they need to address their problems themselves.

Edit: to be clear, I'm no saint. Sometimes I'm the grumpy one and my wife is the paitent one. It works both ways.