Depression Support Group
Depression is a real and debilitating condition that is often misunderstood by family and friends. Its meaning can range from a prolonged period of sadness to an actual mental illness with specific symptoms. Find and share experiences with others who are going through the same struggles.
Also, Simpsons is right; his cheating isn't about you, so don't blame yourself, especially before you know his side of the story.
In the end, this is your relationship and you have to be the one - along with your boyfriend - to figure out how it will move forward.
Best of luck to you.