Random suicidal thoughts (potential trigger warning)

This is hard to explain so this post may be confusing at parts. 
For about a year now, I've experienced random spurs of severe sadness. The random spurs of severe sadness last for a couple hours to a month. The worst part is, I never have any valid reason for this random deep sadness. The extended period of sadness do come off as depression. I lose all motivation to get out of bed and live life. I lose interest in things I love to normally do. The worst part of this all is that I could be so unexplainably happy then it hits me. I push everyone away and lock myself up because interaction with people is the last thing I want to partake in. I randomly think suicidal thoughts and thoughts of hurting myself. Then I go back to being my normal self without the sadness. The suicidal thoughts hit me at any second of the day even if I'm happy and not upset and I don't want this anymore. I don't want the sudden sadness or random suicidal thoughts. I cannot control it. The first time I tried to discuss this with my mom I was told it was all in my head and that I just wanted attention. I feel so alone and like I have no one who will understand. I go to bed in tears some nights because the thoughts become so loud and uncontrollable. I just want answers, what should I do? Who can I talk to if my own mom doesn't even believe me? Any answers and help is greatly appreciated ❤️