How do I tone down my insane jealousy?

I have morbid jealousy and it seriously hurts, really hurts. The thoughts that consume my mind, the intense anxiety that I actually feel in my body, the nausea.
I don’t voice my feelings to my s/o, only when it has got so bad that I have had breakdowns.
It has all come to the surface again today, from how I am feeling, I know it is going to get bad.

Is there anything realistic that can put a stop to this before it escalates? Reading books and going for walks are too mild an action for the level I am at.