When someone says to you, "I love you, but I have to let you go," it means that they love you and have feelings for you, but simply can't be with you anymore. It means that they're very torn about being with you. They can't come to terms with certain things about you and your relationship together. Despite the love that they feel for you, they feel it's best to both go your separate ways. When you're the person that gets left, there's a million thoughts that go through your mind. When you find out that the person you're crazy about is willing to leave you, even though they love you, it can be very confusing. Nothing pulls and tugs at you more than that. It can sound very conflicting to you, almost like they love and hate you, at the same time. Just knowing the fact that the other person truly loves you, but is willing to let you go can seriously be a tough pill to swallow.
What you need to remember and know is that the person that left you, is not the right person for you. The person that's meant to be with you, will stick around through the ups and downs. When you disagree on things with the right person, they'll fight with you, but they'll also fight for you. They won't leave you, despite your imperfections or for ways you need to improve yourself. It may not even be about improving yourself, but changing things in order to make your partner not want to leave. People always say that there's someone out there for everyone, and I know we all get tired of hearing that. But, the fact of the matter is that it's true. There is indeed, someone for you out there that will stand by your side, not judge you, hold your hand, and be a shoulder to cry on when needed. Your true love is awaiting you and when someone leaves you, they're just making room for the right person to come into your life. Keep your chin up and have faith. Believe that good things will happen, and they will.