The boutique owner has admitted he's devastated at what he's done wrong and vowed to win Bobby, 27, back following their split in Marbella.
Viewers of the ITV2 show saw Bobby in tears and heartbroken when Harry admitted he'd slept with his ex, despite previously insisting nothing had happened.
Bobby also found more recent text messages between Harry and the unnamed ex on Harry's phone, insinuating that Harry still had feelings for him and their contact had continued after that initial encounter.
Harry, 20, then ended up catching an early flight home to Essex.
Speaking to the official TOWIE website, Harry said: "I know it's no excuse whatsoever but me and Bobby were only together a week. I've made the biggest mistake of my life."
An upset looking Harry said although he'd done wrong, he was determined to win Bobby back.
"I will get him back because I love him that much and I will prove to him how much I love him. I still regret it to this day. I wish it had never happened because I do love Bobby a lot and I have a lot of time for him."
Harry also tweeted last night: "Just watched last night's show and I am very gutted, what I did was wrong but all I want is Bobby back."
Bobby, meanwhile, tweeted this morning to say he was "heartbroken" and told the official TOWIE website that he can't see him and Harry reuniting right now.
"You can't make someone love you and if I wasn't making him happy – which clearly I wasn't or he wouldn't be in contact with an ex-boyfriend seven months into a new relationship – it's time to let him go."
He added: "With the age difference, maybe he needs to get things out of his system. We're clearly not on the same maturity level, which is a shame."
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