I have said before to take the good things and learn from the bad things. I stand by that. If you don’t learn from your mistakes, all you will do is repeat them. That being said, it is important to treat each relationship as a new beginning. When I say to leave the past in the past, I am referring to treating the person you are currently with as you would if they were the first person you have ever been with.
Whatever happened in your last, or any previous relationship, the person you are currently with has no say or control over what happened. Maybe previous relationships ended well or maybe they ended horrifically. Either way, you need to learn to move on. The person you are with is not your ex’s. If they are one of your ex’s but you decided to give it another chance, this would still apply. Whatever the reason, and whoever you are with, this is an entirely new experience and you need to treat it as such.
I am not saying to forget what has happened. Learn from it. Allow those experiences to influence what it is you are wanting from the next relationship and leave it at that. Maybe your ex was a liar or a cheater. Maybe they were dismissive or abusive. Those all really suck and I am sorry if that was the case. The key word there: was. Be with the person you are with now and remember, that wasn’t their fault. Don’t punish them for it and don’t hold it against them. Let the new relationship grow unencumbered by the tragedies or hardships of the past.