Bad day

Today I totally lost it with Lucas. I am at my wits end and really don't know what to do anymore. Today Lucas pushed me to the limit. And I am ashamed to say I retaliated. I took Lucas over my knee, took a plastic toy that he threw at me and spanked him hard.
Fred came home at lunchtime and found me crying in the kitchen while Lucas was playing on his own in his room. Just before Fred came back, I'd calmed down and explained to Lucas that I didn't mean to spank him and that he'd gone too far with his tantrums this time.
This is an accumulation of the stress from the past 4 days - Lucas has, once again started his tantrums. This time, he throws anything he can find when he gets angry (when he asks for something, like cereal just before lunch and I say no) including sweeping all the cutleries from the table, toppling chairs and today he took the cereal box from the pantry and emptied it all on the floor. When Lucas gets angry he has also started hitting me. All the time outs which used to work is no longer working. When I tell him to sit in his chair in the naughty corner, he sits for a while and then he gets out and plays by himself, like he's not being punished.
When he empties out the tea bags or the sugar cubes, I tell him to pick them up, he walks on them instead and runs away. He defies me everytime. When I tell him to stop when he's doing something dangerous like climbing on a chair and opening the window, he starts a screaming fit.
Fred says Lucas needs to get out of the apartment and run around to expand his energy. This morning I took him to the playground for an hour, and thought he would calm down when we got home. Instead, the minute I drive through the security at our apartment he starts screaming. It was hard enough to get him into the car when it was time to leave the playground, its harder to get him out of the car and juggle with opening the security door and climb up 2 flights of stairs with a wriggling screaming toddler in my hands. When we finally got into the apartment, he started throwing whatever he could get his hands on to show me that he was angry.
The final straw came when he asked for cereal and I said no, since we were 10 minutes away from lunch. He threw the cereal on the floor. Then when I was cleaning the floor, he toppled the chairs. AND he swept his drinking glass off the table. I took him and deposited him in his room, and told him to sit in his naughty corner. Instead of doing what I asked he took his books and threw them on the floor, and took his toys and threw them at me when I opened the door. That's when I saw red and spanked him.
Now I am miserable and feeling guilty. My heart is breaking because I hate having to spank him. But I really don't know what else to do anymore. I want to be a laid back mom, cool and relaxed and happy and just enjoy my baby. I really do. Instead I am strung out and angry and frustrated.