I like one of my friends, but he and I are already in committed relationships. How do I get rid of the feelings I have for him?

Can I recommend you distance yourself from him, and spend more time with your partner? This might sound harsh, but I've found distancing myself from people I have feelings for that I don't want feelings for has helped a great amount. I stopped messaging them unless I absolutely had to, and stopped seeing them unless I bumped into them. Also, try focusing on their negative aspects. Not to make you hate them, but to put you off the idea of being in a romantic relationship with them, and focus on the great things about your partner. What made you fall in love with him in the first place? Things like that

I'm not sure it's possible to "get rid of" feelings ever. I think the most effective and healthy thing to do would be to set up boundaries around the friendship which avoid temptation or things that make the feelings increase in any way. Some distance for a bit might be your best bet. Direct your attention and energy back into yourself and your relationship rather than him. Also ask yourself if you are truly happy in your relationship if you are having feelings for someone else. Maybe it's not the right place for you or maybe it just needs some tending to.