If Pisces lost interest in you, you didn't give him the confidence he needed.
If you know Pisces, you already have to know that he is a very sensitive person.
Pisces usually have difficulty healing and rebuilding after being betrayed and having their heart broken. Pisces feels twice as much pain as others, and it is much more difficult for him/her to get over any breakup. That betrayal or that break is something that marks a before and after.
Pisces will never forget all the damage that was done to him and all that he suffered.
You remember perfectly the pain of being rejected and betrayed. After all this, you have difficulty trusting someone again, which is the main problem. If you want to enter his heart, you have to give 100% of yourself and even more. You have to make Pisces believe in love again and trust someone again.
If he lost interest in you, it is because he was afraid that you would hurt him as others did before. You weren't up to the task, so he lost interest. You didn't give him enough reasons to trust; in the end, what you didn't want to happen.
Pisces is incredibly caring to let someone into his heart and much more after everything he has lived through throughout his life.
She fears letting you into her heart because she does not want to be hurt again. If you want to love Pisces, you need a lot of patience and a lot of compassion.
You have to show him that you are willing to give everything of you to make him believe in love again.
You must make him see that you are in this because if not, he will close the doors of his heart again.