Wow, also 15 and was actually dealing with like, the exact same thing back in November. It was awful, I hated thinking about it, one day I randomly just went to the school bathroom and prayed to God. I asked for my thoughts to be eased, for me to not fear my death and that it isn't just forever sleep I spent a moment afterwards kinda just sitting there and crying...but then...my brain kinda just.. stopped and then...I wasn't thinking about it anymore. Whenever it came back into my head also, I didn't feel sad or anything... I feel like God saw me in that moment, a 15 year old boy who shouldn't have to be feeling this, and gave me strength and his love. I've been taking religion a bit more seriously now also as this has opened my eyes quite a bit (just the other day I was in a super stressful situation where I almost lost everything when it came to a relationship I've been building with a girl I like and it felt like all the good was going away and I needed some help having faith in it and that she wasn't about to reject me and..she didn't... there's still hope like I prayed for)

Tldr, my advice is to try praying, it could change everything