You have a lot to think about. You have to be honest. Even though you are not having any chemistry is there a reason why? Have you tried counseling / communication and finding out why? Staying in a relationship for sake of not hurting the other one is painful for you as well as her. You both seems to have drifted apart anyways. Now that your girl friend is having an affair there is a lot to risk here. The thought of being alone should be secondary please be true to your current partner and also make the choice of letting go and seeing a resolution. That will help each one of you. There are too many hearts breaking here.

I agree with darkside2276, there's a LOT of hurt going on. I would questions how there is no "chemistry" but still in love? There's something missing here.

For one - staying with someone just because you don't want to hurt them isn't doing them any favors - it hurts worse. Secondly, staying with her just to avoid being alone is just selfish. In the end, she's going to get hurt either way. The first thing you need to decide is if and why you want to stay. If you're in love with someone then chemistry of some kind should be present. You said yourself you love your partner as a friend. She deserves to be loved the same way you love your girlfriend, and if you can't give her that then you need to love her and respect her enough to let her go so that she can find someone she will have chemistry with. So, here is what you're facing: you can try to go couples counseling with your long-term partner, and I promise that if you're 100% committed to rekindling the passion with your "wife", you can do it. However, to have true passion and intimacy, you're going to need a foundation of trust, which means that if you do decide to try to go all in you are going to have to tell her about the affair - and it will devastate her and it will take a couple of years to rebuild trust - if she decides to stay with you. .If you don't want to stay with her and really try to make it work, I recommend not telling her about the affair and just leaving on the basis that you're not happy/no longer in love. There's no point in telling her and causing her that pain and then leaving her to deal with it. I'm not judging you and I understand that this is difficult for you. You do, however, need to acknowledge that you are not handling your relationship in a respectful, loving manner - and your partner of 20 years deserves better. I know your gf is seeing someone, but did you really expect her to wait around forever? You're halfway in one relationship and half in another; neither of these women are getting the love and respect they deserve in a relationship - which is why the gf is finding it elsewhere. My best advice is to really search your heart and decide if you want to make things work with your long-term partner. If you don't, the kindest thing you can do is be honest with her about the way you feel and let her go. There is no truth that hurts worse than any lie. You should also try to see a therapist to get the root of why you've continued in this behavior, and more importantly, why you are letting your fears control you. I wish you the best and I hope that you can muster up the strength to be honest- with yourself and with your partner(s).

Thank you for your comments. I'm struggling with my life right now.
I think I know that I need to talk to my longterm partner and split up. My gf doesn't know if she wants me or not now. How can I convince her she's the one I want and need? She's pushed me away because she didn't want to get hurt if I never left my longterm relationship and is strong enough to not need me.
What a mess.