Hello Black Forest
Welcome to the forum. Does your name reflect how you feel? If so that's a pity. I have to say I like black forest cake, what about you?
It's great you are doing something for yourself instead of for others, especially when it is at your expense. May I ask, what do you expect from coming here? I am asking because I wonder if you are posting here in the same way your friends tell you their troubles. Sometimes it helps to realise what others want but also how much you can give. Here we don't tell you what to do but we do offer our own experiences and ways we have managed our various difficulties. It often helps to know there is more than one way to manage our lives and that it's OK to try different solutions.
You said, 'then has a meltdown days later, because I can't help them or make them happier' It is not your role to solve their problems or make them happy. People have to take responsibility for their own lives while asking for help on their own journeys. We can give you all sorts of ideas and activities that have worked for us individually, some of which may help you,. We cannot tell you what to do.
Please do not take this as a brush off. I am trying to compare your reasons for coming here to your other friends reasons for confiding in you. By the way, it is a compliment that they do feel safe confiding in you. If you tell them what to do and it works they will come to rely on you for answers. And if it does not work they will blame you. Neither is a good result.
By all means listen but let them make their own decision. If someone asks your opinion ask them what they want to do or happen, then ask how they can do this. Now you are probably laughing at me telling you what to do when I have just said people must make their own decisions. However this is not quite the same. It is a suggestion only and one way to help your friends decide what to do.
I see you feel a bit abused by these folk who only speak to you when they want something. That may well be the case but it may also be because they trust you and value what you say. You say you have a meltdown when you worry that you are not helping them. 'They trust me, I would always think, But, if they trust me, then what would they do if I can't fix the problem?' You can do nothing. Please remember it is up to each one to decide. It really is not your job. For your own peace of mind please remember this when someone unburdens themselves to you. That's OK but not your problem to solve.