Wht can't I be happy

On the outside i have a great life. I rock climb and have an ok job and I'm doing 'ok' by most standards. Inside I feel so dead. I'm not new to depression, I suffer from BPD and major depression already but out of nowhere this bout of really bad darkness came on and it's awful. I feel alone and worthless and trapped in my life that brings me nothing but sorrow and bits of happiness and then snatches those away as quickly as they came. You know how people say what doesn't kill you makes you stronger? I think it makes you stronger in the sense that finally one day you're strong enough to pull the trigger. I've made a plan to go, seeing it through when I'm ready is just the last step. I just feel like I have nothing left to give or live for or like my time is up and all the potential I had is gone. I feel so hopeless and like my future is nothing but a repetition of pain. I've cried so much this week and I just don't see any light at the end. I'm tired. I'm ready to just let go.