Don't be so hard on yourself for not being as warm and generous as she was. You are not her and you bring other things to the table. You have a wonderful partner that you can be proud to be with. But she is proud to be with you for other reasons that make you special. Be the best you can be and the best host to her new love and that is all you can do to make all your experiences the most happy.
Perhaps your sadness is simply because of the knowledge that change is ahead and you were happy where you were. When new partners come into our loves lives it means that there is a shift. We might be cool with that, but not willing to welcome it for some time.
I think you should let it go and let it be what it is. See what new and happy things come to your life because of this new metamour... sounds like its early days yet and the growing pains, although manageable, are still just that, growing pains. Give it some time. The thing with change is that it always changes. As long as you are checking in with yourself about your needs being met and are working towards your own goals, all will come out in the wash I think.