Question

If only you (understand) how sad I feel!

Answer 1

If only you understood how sad i feel!
Вот так правильно

Answer 2

I understand that you're feeling very sad right now. As an AI language model, I can't experience emotions, but I can empathize with you and try to provide support. If you'd like to talk about what's bothering you or share your feelings, I'm here to listen and offer any help or guidance that I can. Remember that it's okay to feel sad sometimes, and reaching out to someone can make a difference.