I was thinking about this again just now and how unhappy you sound. I've been there. I left my gorgeous big house for a tiny new place just cos I couldn't bear it anymore. I recently found mould in our new flat last month and almost freaked out but managed to reign myself in. So here's my tips:
-splash out on the best dehumidifier you can. Ours is a Meaco 20L from John Lewis. We run it 24/7 and simply accept the running costs as part of our rent! In turns off when it hits the desired humidity then checks the air every 30mins and comes back on if needed. (We've not had the heating on once since we got it, when we used to have it daily, cos it takes the cold out of the air too! bonus)
-get a tumble drier if you can. We use it 99% of the time. Kids uniform dries on an airer over the bath.
-Keep bathroom and kitchen door shut ALWAYS and the windows open when those rooms are in use, and afterwards. However, open all doors in the entire house while everyone is asleep to allow air to circulate.
-No furniture against outside walls. This has meant for us having the bedrooms not look how we would like, but the sacrifice is worth it. Leave 6 inch gap if you must have furniture there.
-fling windows open wide as soon as you wake up for half an hour if you can bear the cold
-keep heating on a low heat always rather than putting on and off
-Use Dettol mould + mildew remover in green bottle as soon as you spot any mould.
-Do mould checks! We did ours every 2 weeks in the old place- pulled out all furniture.
-less stuff. Be ruthless! Buy 'simplicity parenting' it really helped change how I thought about stuff.