How do I care for lonely or difficult clients without burning out?

You have grown into this. You can connect with most of the people you care for. What drains you are the lonely ones who need endlessly, and the difficult ones who push you away. Those two take the most out of you because they give the least back. Feeling depleted by them does not make you a worse caregiver. It makes you an honest one. Hold this gently. The lonely person's bottomless need and the difficult person's sharp edges are usually the same thing wearing different clothes: pain, and fear of being left. It is not really aimed at you. Knowing that will not make the days easy, but it can stop you from taking the drain personally, which is half of what exhausts you. So guard your own fuel. You cannot pour from empty, so build one small refill into your day that is only for you, a walk, a quiet coffee, ten minutes of silence. Protecting yourself is not selfish, it is what lets you keep showing up. You give a lot. Make sure someone, even if it is you, gives something back to you.