Too tired to help my best friend

I have been feeling my lowest for a couple of months now, so I quit every project I work on last week and decided to take a long time off to try to recover and just focus on myself and get better.

My best friend doesn't know about my depression or about my current situation, she broke up with her boyfriend recently and I have been trying my best to be beside her and make her feel better even when I felt so low, including last week, I talked to her for hours and listened carefully, I comfort her and I didn't close the phone till she felt better and again I was too tired, and after the call I felt worser because I felt like I didn't help her much and didnt do my best and I didnt feel like I deserve to be called a friend.

Two days ago she asked me again for help I wasn't feeling good at all to the point I couldnt talk .. so I lied and told her that Im busy. And again today she called and I didnt pick up, I cant, I feel so fatigue and listening to more problems makes me more tired.

I feel tired when I talk now, or communicate, I dont have the energy to listen or talk or help anyone even if I want to, I want to but I cant and this kills me because shes such a good friend but Im not and I dont know how I can help anymore.

So did anyone felt that way before? Do you have any advise on what to do? I feel like Im going to lose her soon, because Im not beside her anymore Im always away and running from listening to her. I want to, but I cant, its exhausting, and Im exhausted.