Insulating a former alcove space to a closed off wardrobe

Hi, We had an odd alcove space in our upstairs bedroom , too small for an ensuite so we fitted a door and closed off the space mocking a wardrobe style inside (just railings) Now one side of that wall itself is facing the front of the house (south facing garden this the front is cold) So by closing this space we've now suffered damp (light flecks of mold) and condensation Ideally I don't want the whole work to go to waste so what can I do next? I have since removed the door and now it's just a closed off space with a door missing :( 1. How can I effectively heat up the wall or offer ventilation (door is already removed) I thought to drill holes in the cold wall but then it'll suffer from draughts 2. Will this happen frequently as it is an exterior part of the wall so knock down back to an alcove space? 3. I can paint with zinsser anti mould paint but is this short term and just papering over the cracks? Any other suggestions welcome Thank you in advance