My brain feels very heavy and clogged. it had become difficult to concentrate or even think straight.

Consider Depression: It would be wise to check with a physician to rule/out underlying medical problems. If no clear answer emerges, consider meeting with a qualified and licensed psychotherapist to explore psychological contributions such as depression.

What's the issue?: Your general symptoms can be due to several issues, alone or in combination - anxiety, panic, depression, attention-deficit disorder, OCD. It would be helpful to visit with a counselor or psychologist, who can help identify the components and break the negative cycle you have now found yourself in.