Help! I have a relationship issue...

I am a man in my early 60's living in the UK. I have been happily married for the last 40 years. 18 months ago I met a woman (purely by chance) who I became highly attracted to. I assumed it would just be a transient attraction fed by a perhaps predictable desire for 'something new and exciting'. However, I fell in love with her and I don't know what to do. I never looked for this and in many ways I still love my wife. However, the woman I met seems to make my life so much richer and more satisfying. We are very alike. After 18 months it's clearly not just a flash in the pan. Yes, I know many of you will want to judge me but I would really like to converse with someone who has also been in this situation with helpful advice. Help!