I found 2 love letters to H. One had his name on the envelope so I will assume that one was handed to him personally. The other one was mailed to him here.. but to his company name.. it looked like a business package when it arrived.. he gets tons of business mail, I (obviously) do not go through anything for his work. It was from a woman in Germany. (He has been on 3 business trips there since winter)

Anyway, while I suspected infidelity for a long long long time, (separated for 4 yrs in same house) H always professed to be not dating. (Though he made no efforts on communicating or working on marriage whatsoever).
(Is it even infidelity if separated but in same house for 4 yrs????? Most people would say this is certain to be expected I guess?)

Anyway, I suppose with the copies of letters in hand I can add "Infidelity" to the reasons of filing for divorce legitimately. Should I even mention this to H or just let it go at this point???

What I would do is show it to your lawyer first. Then show it to your adulterous spouse. If possible, make a copy first, then give him a copy.

Just a thought, but in the long run who cares. Copies or originals. The affair is out of lust and unthoughtfulness. There was nothing else to my stbxh affair. Nothing at all. He was used by a promiscous woman, with no morals. She knew exactly what she was doing, since this was her 2nd affair for sure. And she knew where she wanted my husband. In bed with her.

Just move on, and get yourself together. You sound strong and capable of moving on.