Enter the relationship with realistic expectations. If your partner just got out of a serious relationship and feels utterly heartbroken, he or she could be using the love you give them as a means to cope -- or to avoid coping -- with their loss. Ask them to take some time and make sure they're really ready to be in another relationship. If you're already together, you may need to take a breather until they are in a better place.
Expect your partner to experience a range of emotions, both from grieving over their lost relationship and celebrating their new one. Don't expect them to forget about their ex overnight, and don't take it personally if they miss their ex on occasion. Those feelings don't necessarily mean they want to be with them. They just mean they are experiencing the normal, healthy process of letting go.
Establish a boundary for how much you're willing to process with them over their ex and call them out if they don't respect that boundary. Talking about the past may be OK, or even welcomed, once in a while, but using you as the shoulder to cry on every day could lead to "not one, but two failed flings," according to Fox News' Oliver "Ali" Nejad.
Avoid serious commitments at the start of your new relationship. According to dating coaches at The Relationship Gym, "because [he'll] be feeling more vulnerable than ever, and more eager to retain the love [he's] receiving, [he's] even more likely to over-commit."
Adopt patience as your personal mantra, but put yourself first. There's no way to know how long they'll be in rebound mode. How long you remain patient is up to you, but "if you feel belittled or hurt on a constant basis then he is not worth the distance you can throw him," says relationship expert Natalie LieLue of Baggage Reclaim.