^^I love this post so, so much.
GalaGirl is absolutely right. Nothing is preventing you from loving your friend and having romantic feelings for her. I love someone as definitely more than a friend and we are not in a sexual relationship. You are not cheating on your wife simply by having feelings for someone else (although some monogamists do like to froth at the mouth about "emotional affairs," which is bullcrap, IMO). Nor is your girlfriend cheating on her boyfriend by having those feelings. What would turn this into a cheating situation is seeing each other in secret with the intent to have sexual contact. Can you live without that to honor your agreements?
If the answer is no, then I'd advise you to keep talking about polyamory with your wife. There may be a way to move forward towards an open marriage even if she got spooked once. You say she cheated, but you didn't actually cheat, just intended to do so? I'd say you two still have a lot of trust issues to work on. Polyamory generally doesn't work well for an existing couple unless their trust is very strong.
It is perfectly possible for a couple to have an open relationship where one person is emotionally poly and one is just out for a good time and doesn't want emotional ties to their lovers. You don't both need to have the same relationship orientation for this to work. But you do need to have good trust between you.
Maybe a couples' counselor with some knowledge of polyamory could help you navigate this? You may be able to find one here: