Most Helpful Girl
Firstly, her fling from 25 years ago shouldn't be the issue. You were both young. You built a relationship despite that.
This time though, an affair that lasted 7 months? No way she could ever have thought it was even remotely ok. Sleeping with someone once and admitting it, I might be able to forgive. But an extended affair, sneaking and hiding and lying for months and months? No way.
Most Helpful Guys
There seems to be information missing. Wonder why she called it off if it seemed to be going alright with the other guy? Did she come back to you for financial security? Why did she feel the need to have an affair in the first place?
Oh well, who knows, only you can decide whether to continue the relationship. Don't just end it because of your ego, you have to think about whether she genuinely wants to rebuild the relationship, or if she is staying for other reasons.
Fool me once, shame on you
Fool me twice, shame on me.
I don't have much to say about it except that divorces are expensive. If you want to go through with it. Start selling all of your stuff to a trusted friend. Cars, boats, tools, etc. Even though she was the one who did the deeds, the magistrates usually look in favor of the female. If you want to continue with your marriage, then some couples counseling may be in order. It sounds like she got dumped by the other man and is keeping you just as a matter of convienience. I wish you well.