Velvet I know exactly where you're coming from, your feelings match mine completely. Everyone being so excited is frustrating, and you end up wanting to scream that no-one understands how you feel.
I also hoped i'd feel better after 12 weeks but like you, it didn't feel that signifiant. I'm now being pressurised to go fully public at 20 weeks but still don't feel comfortable doing that, esp as I still have no bond with the absent bump.
My very basic advice is to do what makes you feel calm, and less mentally, not what other people want you to do. Your mum will have the best intentions but being told to enjoy it won't help you relax.
I agree that some forward planning may help, altho I think you need to be careful. I can't face buying anything, or making decisions, but can cope with accepting offers of 2nd hand stuff (it helps I love pre-loved/antiques!). This may help you. I'm also thinking of asking the sonographer to write the sex down & pop it in an envelope in case I feel that knowing that will help me bond in the next few weeks.
Lots of hugs to you as I know how it can feel. If you're starting to feel very unhappy or disconnected, pls see your Dr or MW for a chat.