Assure him 100% that you won't **** things up on your side. The guy isn't perfect either so he'll probably do things to set you off, but you two will have to remain open with each other. The main reasons relationships fail are because of lack of communication, lack of emotional support, and problems in the bedroom. If you have those 3 things going for your relationship you'll never have any problems.
Things are looking pretty bad for you though since there is a new girl in the picture. He is already starting to move on and you're trying to bring him back. Make it worth his while. Do something to show him that you're better than the other girl. This might require stepping out of your comfort zone, but that's how you grow. So if you want him back you have to do something along these lines.
Cheers
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