It sounds like you have a kind heart but it also sounds like you are struggling with setting and maintaining boundaries. Your adult children are, to state the obvious, adults. Regardless of the circumstances and their reasons for moving in with you, housing your adult children is not the same as providing a home for children who are young and truly unable to live on their own.
There are several practical options available to you for getting a handle on the situation but all of them require a willingness to treat your adult children as adults, meaning that you must give them the responsibility to solve their own problems. If you are not able to accommodate all their “stuff” they have to figure out what to do with it. Fortunately, they have several options.
They can get part-time employment that allows them to make enough money to rent storage for their belongings. It is possible to find storage units that are not ridiculously expensive. They can downsize some of their stuff—that is, sell it, give it away, donate it to a charity or ask other friends to store some of it in their garages and basements until they get resettled into a new independent living situation.
Your part is to determine what it is that you need: what portion of their belongings you can reasonably accommodate and what you cannot and not to beat around the bush with them about your needs and requirements. If is situation is affecting your mental health, it is not acceptable and has to change. Your adult children are part of the total equation, but so are you. No one can be expected to respect and value you if you do not respect and value yourself. You have the right and the obligation to yourself to insist that they find a solution to the problem of their stuff. It is their problem, not yours.