、。
I'm tired, and on top of that, the exam is coming up.
Literal
I tired-[state-ている]-[reasoning-し], on-top-of-that exam [subject-が] approaching-[state-contracted-てる]-[explanation-んだ].
Several casual features. 僕 drops the topic marker は — common in very casual speech. ~し is the soft reasoning/enumeration particle meaning 'and (what's more) / for one thing,' used to list contributing reasons. それに ('on top of that, moreover') introduces an additional point. 近づいてる is the casual contraction of 近づいている. ~んだ provides explanatory framing throughout.