Can I just take a minute on my soapbox?
Alright, here's the deal: breakups suck. They really do. There's no way around that. But I think what would suck more is to find out that my ex wasn't honest with me about why he wanted to break-up.
Yeah, I get it. Honesty is terrifying, especially when you know that the breakup will really hurt the other person. But, being honest with them, although painful in the short-term, could help them realize some things about themselves that they can't see on their own and help them prepare for their next relationship. Plus, it's cliché, but true: honesty is always the best policy.
So, if you've started noticing other people and you think that you and your sweetie need a break, tell them that. It's so much more respectful and noble for a breakup to come that way than to find out later the real reason behind the breakup. Plus, talking about the change in your feelings during the breakup will help cut any awkwardness if you get together with someone else really soon after the breakup. (Notice how I said cut, not demolish. Awkwardness will still be very present if you have a new relationship one-week post-breakup.)
The best way to decide what to say during a breakup is to imagine yourself on the other end of it. If you'd want to know the real reason why, your soon-to-be-ex likely will too.