My cat died last night

My cat died last night, and I don't know how to move on. She was just 3 years old. The only symptoms she experienced was difficulty in breathing and no appetite. We even brought her to the vet yesterday morning. They run cbc test and found out she is low in platelets. Her RBC and WBC are normal. They just prescribed her thromboost and doxycycline. We insisted on putting an IV to her because she doesn't have an appetite. We took her home and give her the meds but after an hour she took her final breath on my bed. I don't know how to process this because I don't even know why she died. I don't even know how to put into words what I feel. I am very very sad.