I don't think it can ever be a mistake to be honest. It's pretty unlikely that after 5 years you wouldn't feel "that way" about him if you could. Do not let him guilt you about this. It us understandable he is let down and feels like this came out of left field. That might be his deal-breaking line there - "I'll be with you if you don't live with/marry your girlfriend, and as long as you love me most" You know that this isn't your truth, so all you can do is hope that he is more willing to talk after he has some time to think, and you can salvage different but more honest relationship with him out of this.
Now it's a separate issue if you think he or her were "swaying your feelings" if either of them were talking negatively about the other that's an issue to deal with. If you're just too unsure of yourself to have a strong identity to know what you want (which sure might be an issue since you struggled with this so long) that's an issue you should deal with ASAP, certainly before you start dating anybody else, as in counseling, read about self esteem, making a great effort to be 100% honest when you are asked a relationship question by a partner, even if you think it's not what they want to hear, whatever.
I'm sorry you aren't happy with his reaction, but I really think you did the right thing, and I'm sure you've learned a lesson or two about the importance of being upfront that will serve you well in the future.
Happiness will never come to those who fail to appreciate what they already have.