starting on mirtazapine and first night was horrible!

I'm currently on citalopram 20mg and my doctors prescribed me 15mg of mirtazapine and I hated it!! I felt tired very and uncomfortable soon after taking it and was dazed and dizzy and slept for like 12 hours whilst only waking up for 2 hours before I took it. If was awful! I only took half a tablet at the night time but I hated it so much I'm scared to take if tonight. Are the side effects worth the outcome? I doubt ill ever take it again to be honest.