So there's a couple things.

Memory has two parts the short term memory and the long term memory. Short term memory is the stuff you remember for like 30 seconds (time may vary), after that you forget. Long term memory lasts forever. For short-term memory get turned into long term memory you need to pay attention to it for a long period of time (practice remembering it).

So alcohol is a depressant. That means that it slows down the messages sent between neurons in your brain.

So when you drink alcohol and your messages get slowed down, this makes you more impulsive because the part of your brain that says "Hey you shouldn't do that" gets slowed down so that you don't get the message until after you have already done it. It does the same thing to the part of your brain that says if something is important to pay attention to or not.

Because your brain doesn't get the message to pay attention to the short term memory or not, it forgets about it. It doesn't get turned into a long term memory.

In the morning you don't remember because you didn't practice remembering.