Closed Communion?

If you didn't know the rule forbidding you, then it was not a mortal sin. One of the requirements for a mortal sin is that it was committed knowingly.

You should avoid taking communion anywhere you do not agree 100% with their doctrine. Part of taking communion is saying "I believe in what this congregation believes; I am a part of this community." If you're not part of that community, don't take communion.

Of course! Though I find it hard to believe you can't find a single priest anywhere to hear your confession.

To pray to God for forgiveness? Not really.
To confess to a priest? Yes, but it's different denomination to denomination.

There are different Catholic and Orthodox rites, but the differences are cultural, not doctrinal. Don't worry about them.

NO YOU CAN'T user!
it is forbidden. You are in mortal sin. The mortal sin of heresy.
I'm sorry but there cannot be intercommunion between denoms.
I'm not sure that that is true, but I've heard some cardinals that were going nuts over some of the Pope's actions.
Tldr: don't do it.

Fix your life.