“I feel tired. I’m not getting things done like I used to, I’m not interested in friends and family or going out for things I used to enjoy. I can’t concentrate. More and more I feel nothing good ever happens to me. Life is not worth living. How can I feel better? I just want peace.”
I could say, focus on what you want. What you think about expands. If you want confidence but keep thinking “doubt,” then what you’ll keep getting served right back at you is more doubt, more reasons why not. I have a sense your question is not about this.
So I’m not even going to start about simple state change – i.e. from standing to doing jumping jacks, or even suggest that you break through this by “taking control” and creating new attitudes. What you’re describing sounds more complex; like there’s a combination of energies involved – fatigue, anxiety, boredom, disappointment, hopelessness, worthlessness, sadness, etc.
There could be a number of causes for what you’re experiencing.  Physical illness. Chemical or hormonal imbalance  Unresolved emotional issues  Prolonged stressful conditions. I’d advise you to seek the help of a doctor, psychiatrist, or therapist to know: What’s causing those emotions? What are the triggers? Given your description you’re unlikely to be motivated to find out so why not seek out a friend or family member – let them know you are not functioning well and need support.
Many people I know suppress their emotions; society says keeping a stiff upper lip is what’s acceptable. Oftentimes, we suffer ulcers, anger issues, or other psychological conditions because of this. Perhaps, realistically you may need to experiment with different solutions over time to find what really works to lift your spirit.