TWRP recovery not launching

I have OnePlus 3 (A3003). I installed a custom ROM. Since then, I can't get to recovery mode. If I try booting into it, my phone gets stuck on the OnePlus logo. I tried flashing it with fastboot,

C:\...\Android\android-sdk\platform-tools> fastboot flash recovery recovery.img
target reported max download size of 442499072 bytes
sending 'recovery' (18964 KB)...
OKAY [  0.580s]
writing 'recovery'...
OKAY [  0.144s]
finished. total time: 0.726s

And also I tried flashing it with the TWRP app, it gave me an error telling me that I have already installed it. What is the problem?