Listening to constant crying from a baby and a lack of sleep can render you zombie-like. It is hard to think straight, memory is impacted and perspective becomes skewed. In other words, don't underestimate the effect that the crying and lack of sleep will be having on you physically and emotionally.
Your baby is only 12 weeks old. The last three months will have seemed interminable because of the crying and clinginess but really it's only a short amount of time. You have done BRILLIANTLY to have made it through this period. You think you're not a natural stay-at-home mum but give yourself a break - not many people could get through what you have without climbing the walls; I would be on my knees!
But do consider carefully your plan to end maternity leave early and return to work. I've always worked full time and juggled that with being a mum of three so my advice is to bear in mind the following:
-who would look after her if you went back to work? Would you really feel comfortable leaving a 'high-needs' baby in a nursery or with a childminder?
-how would you cope with a working day when your sleep is so disturbed?
-the hardest part is behind you! It will only get better from here on in. Once she is rolling, sitting, crawling, cruising - which isn't far off! - things will change beyond all recognition.
-have you thought about going back to the GP? Has she been diagnosed correctly? I have had no experience personally but I have read good things about cranial-oestopaths that you could try.
-if you've found expressing hard and she won't take a bottle anyway, then she really needs you there for breastfeeding. You are half way through the EBF stage already! In a few months you will introduce solids and the BF will reduce.
-it sounds like you're missing adult conversation. Could you invite a friend or two around to your house for a take-away one evening when your husband's there? That way you get the conversation without the stress of going out.
-if you want to go out with your baby, do it! Don't let her crying put you off! A walk everyday really helped me feel better, especially in the summer months.
Good luck whatever you decide! 