I hate that magical answers don't exist, but, yeah, they don't exist. But Thunder Bow is right, get a therapist who can help you carry some of that heavy baggage.

It's okay to feel tired/depressed/lonely sometimes. You don't have to force yourself to be happy all the time to be a good person. Seems like you're carrying a pretty heavy burden, taking care of your grandfather. That's a wonderful thing to do for him and I'm sure he loves you for it. But being a caretaker is such hard work, so you have to make sure you're caring for yourself as well. Even if it's just taking a walk, or a fifteen minute nap, or reading or eating chocolate- it's okay to want, need, and give these things to yourself. Positive self talk isn't "I must be happy NOW," it's "I am struggling. That's okay, but I would like to be better. What is actually going to make me feel better?" And it's not easy- just like any other habit, your negative thoughts are hard to break.

Try to stop punishing yourself. You're a good person with a lot on your plate. It's well worth it to take good care of yourself.