Why do they still happen?

I am 25 now but I had my first seizure when I was 20 years old. At the hospital the CT scan revealed a tumor in my brain which I had removed a few months later. The tumor was classified as a Pilocytic Astrocytoma, which the neurosurgeon said could have been in my brain since I was a child. So after the surgery I was put on Keppra. The Keppra made me very short tempered, I would get extremely mad at the smallest things. So, like a dumby, i stopped taking it one day because I was tired of being mad all the time. Guess what.. I had my second seizure. I woke up in the middle of the night on the floor with a severe headache and my tongue hurt from chomping down on it. I went to my neurologist and he switched me to Trileptal and I've been taking it since. My 3rd and final tonic-clonic seizure happened after a night of drinking, I would classify it as excessive drinking, and I forgot to take my meds that night and that morning. Since then I do my best to never skip a dosage and to not drink. The alcohol does not mix well with the meds nor my brain for that matter. I have not had a tonic-clonic seizure since the 3rd but I do still have focal seizures, I would guess probably 10 of them a year, even with the meds. My motor skills still work, but i cannot speak. My question is, since I had this tumor removed and I don't miss my meds, why do I still have these focal seizures? Was that surgery even necessary? Another scary thing is I remember having these focal seizures moments before I blacked out when I had the 2 seizures when I was awake. I can feel the focal seizures coming moments before they happen. They are very scary because I don't know if they are leading to another clonic-tonic seizure or not. Why do they still happen??