Let me explain this one. At the time, I had the largest amount of simultaneous inbound traffic at KSAN of the whole night, and just this one aircraft for KSEE. I had sent him basically on the right downwind for 27R at KSEE, preparing him for a base turn, then an ILS intercept. Cluttered with other requests and a descent amount of traffic to KSAN, as well as Some other requests, I didn’t even think to watch for terrain around his aircraft. It just completely slipped my mind. I turned him base a little delayed from what I planned due to other requests. He started turning, and then disappeared. I didn’t think much of it, mostly because I was occupied with all the other aircraft around, and didn’t realize to even look at the terrain near there. It was my honest mistake, and I apologize, as I feel horrible about this. I will take blame for this one.