Hey, just to send some support and to say I totally get what you mean... a few things I've noticed to far (30 weeks now)...
Firstly, I and quite a few others I asked found that the transition from first to the second trimester isn't bang on 12 weeks, many of them were still feeling a bit rubbish into week 14. I think all the nhs, what to expects etc slightly generalise and evangelise about the "brilliant" 2nd trimester. Ok loads of people do get an upswing, but I think a lot of others still feel y'know, a bit pregnant and shit! I felt better but not the glorious zen period I kept reading about.... having said that, hang in there, you could yet still have a great time of it.
I have found that there have been peaks and troughs within the trimesters too, I think I can link it to hormone surges as I'll suddenly get all constipated, tearful, sleeping like shit and generally knackered for a period of a few days/week.
One thing I have really noticed is that when I've felt those periods of tiredness and run-downness my confidence to cope, and general mental well being suddenly tank too. I feel pretty useless, demotivated and overwhelmed. In some ways just using a google calendar to note this down so you see a pattern and accepting OK I'm having a shit couple of days and flagging it up to DP and where possible colleagues in no uncertain terms to give me a pass/hand with stuff has helped. I hate feeling useless and zombified though!
I've also just accepted that my brain is literally on half cock the entire time. Making basic decisions, let alone getting on top of a heavy workload which it sounds like you are can be really hard with that and the tiredness! It isn't forever though, so give yourself an easy ride rn if you can.
I'm pretty pissed off that there isn't more recognition that being pregnant is fucking hard work on that specific front. Ok when you get bigger and people see the physical implications there are more offers of help and support/sympathy etc, but I think the other side of it like you're describing is largely ignored or flippantly described as "baby brain", or hinted at as general fatigue etc.
Not sure if all this is helpful or not, but just to say I and loads of others recognise how you are feeling and totally understand, and if in doubt remember 14 weeks is already 1/3 of the way there xx