I want to start by congratulating you on your achievement. Achievement may actually be a big part of the let down you are feeling. It’s not unusual to feel a depression or let down after achieving a long term goal. Even though it is a joyous occasion, there is also a loss that sometimes goes along with completion.
Having a long term goal such as a college education gives a sense of purpose. It’s also something that is achieved over a number of years. It becomes part of who you are and is incorporated into your daily life. When that ends what has been normal is suddenly lost. It’s not as if you can flip a switch and find a new sense of purpose -- it’s a transitional time. Often transitions can be filled with many mixed emotions including; sadness, depression, and anxiety.
I think what confuses most people is that you can feel depressed even though you have just accomplished something great. Sometimes it just takes some time to make your way through the transition. However, if your depression is interfering with your normal functioning level then it’s important to seek professional help. It also goes without saying that if you are having any suicidal thoughts to seek help immediately or call 911.
Even if you are not in crisis or the depression you are feeling is very mild, talking to a therapist can help facilitate the process of transitioning from a college student to the next stage in your life.