Death anxiety symptoms and conditions

I'm a 30 year old male, and I've been on Singulair for eight years now to treat chronic asthma. Shortly after I started, there was a noticeable change in my mood and behavior. What I noticed included sadness, thoughts of death, anxiety, and fatigue. I knew that there was something wrong, but just chalked it up to changing life experiences. I was always heavily involved in organized sports (little league through college), but abruptly stopped after college. I thought that the moodiness (or depression) came on because I wasn't as active anymore. Correlation maybe? I don't know. But I do suspect that Singulair is the culprit. I would like to get off for a while, but I literally cannot breathe without it. Does anyone know of any comparable alternatives to Singulair that won't bring on these type of side effects? Thanks all for sharing. Blessings!