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I know that you can’t bring yourself to say anything but remind yourself that this is one of those things that is better to get done sooner rather than later. I know you don’t want to string him along or pretend everything is fine and it’s also not fair to you to be stuck in a relationship where you’re not truly happy with how things are. It’s also better to break up in person than over text if you can help it. Remind yourself of the reasons why you wish to end the relationship in the first place when you go to back down. Sometimes it helps to remind yourself of the reasons why things aren’t working out in the first place.
I’d sit him down on a day when he’s in a relatively decent mood so he’s more receptive to what you have to say. Be honest with how you feel by using “I statements” so it doesn’t sound accusing. Be honest but polite. Perhaps say that you feel as if you two are headed down different paths in life and you don’t feel as if the relationship is working out. Let him know you’re breaking up with him but you’re not mad at him.