I understand why your solicitor is advising you this way. If your wife isn't admitting the adultery, you need evidence.
My divorce was similar and my ex also put unreasonable behaviour. I wanted to counter petition and had no evidence, so in the end as it was obvious that he wanted the divorce, I decided that the divorce could go ahead, although I made it plain that I disagreed with his reasons for it, but would not contest it and this was sent in a letter to him.
It is a very difficult time, and you have been hurt, and want to hurt back - but believe me, you will just drag it out by counter petitioning and it will make no difference to the outcome.