How do I get my mind off this?

I have still been working on getting over my ex. We broke up after a few years due to religious difference and it was extremely rough. It's been 8 months, and I thought it would have stopped hurting.
I had asked her to marry me before she left on a mission and that's when it ended. But it officially ended those 8 months ago. I was feeling better, but still deep down hoped we'd get back together.
I recently found out though she's engaged, and it set me off.
I'm just absolutely devastated, I thought this was the love of my life. For the first few days, I was able to keep it out of my mind. Now I can't. I wish it would go away. But all I can think about is that guy marrying her and having sex with her and pleasing her and eventually giving her a family.
It's tearing me up and every time I think or hear about sex it makes me sick to my stomach.
How do I stop thinking about this and forget her?
How do I get my mind off this?
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