I would suggest you focus on your marriage and exhaust all options to make it work and become best friends and best lovers with your wife.
If the other woman, your best friend, is really worth it, really is a good friend to you, she will respect your decision to honor your commitment to your wife as much as possible. She may have feelings for you, but tire friends don't try to break up a marriage, no mater how strong the feelings are.
When people leave marriages for another person - it often does not work out. It's partly because underlying issues are sometimes ignored. It's also often because both parties know the other may be willing to cheat or leave a committed relationship, any committed relationship, if someone new comes along. It's also because of how the relationship started - as an enticing escape and alternative... Without the realities and problems that usually come later on.
If your best friend is really a best friend, she may be disappointed but will support you spending time exhausting all ways to make your marriage work.
If she is not supportive of this, she isn't really a best friend or a friend at all. Real love sacrifices the needs of self in healthy ways in order to help another be happy. Sometimes love means saying no to things we want and things others want. Love is very hard, but it is worth it to make the right choices, even if it is painful sometimes.
You wrote that you can be happy in both places - and that makes me think this is less about love and more about how hard it is for you to say no. It is common in marriage to be attracted to other people. Marriage is a commitment to not go for all those possible relationships but to have your wife be the one, in spite of other once notable attractions to other people.
You already know you can be happy in your marriage, do the loving thing for both your wife and your friend and tell your friend no to having a romantic relationships with her. If she really loves you back, she will love you for your desire to honor your commitment to your wife. Real love honors and respects such boundaries.