Lord, have mercy.
You will struggle with belief your entire life. Just remember that the struggle is what purifies us.
Regarding sitting in on the other group, this is one of the reasons why the canons forbid us to pray with heretics. You are not supposed to be doing this, so I would advise against any official religious contact with them. Sure, meet and discuss things, but you should not make a habit of meeting with them if it places Orthodoxy on an ontologically "equal" level with their denominations. Remember that Orthodoxy is not a denomination of Christianity. Orthodoxy is the Church Herself, the one which all of their groups were once a part of, if you follow history back far enough.