I'm not trying to say you're silly for being stressed - not at all - I can be the biggest stresshead you've ever met so know how awful it feels. But sometimes a bit of perspective helps. It IS only nine weeks and although nine weeks can drag and feel like absolute hell, if you try to approach it as only nine weeks, then yay you're on maternity leave, you can kind of change your thinking around to this being like 'playing' at being at work because you'll be gone soon. Hope that made sense. Try to think of ways you can have fun at work. D'you know your colleagues very well? Chances are they think the targets suck too. Sometimes a good gossip session works wonders and it helps when you're all in the same boat.
Fiona has raised a really good point here. I totally understand where you're coming from hon (I was over work at the 20 week mark!) but taking each day and each week at a time will definitely help. I know it's hard to believe but your maternity leave will come around sooner than you think. Everyone said that to me and I thought the time was dragging but it really wasn't. Be sure to give yourself something to look forward to each weekend and don't be disappointed when Monday comes around again, just think "I'm a week closer to finishing work and a week closer to meeting our baby". That really helped me.

You should be entitled to stress leave so definitely discuss that with your GP and your manager. Stress is nasty for both you and bubs so yes, best avoid it. Lighter duties is a really good option too. I had a few random days of very nasty braxton hicks about 4 weeks before I was due to finish work so in the end my boss suggested I just take it easy and work from home until I finished! I couldn't believe it although I knew I had an understanding boss. It was so much better as I didn't have to worry about getting all dressed up for the office or the drive to and from (approx 40 mins each way). So if you can work from home at least one or two days a week then definitely suss that out too.

Hang in there and know that you're not alone. It's so tough to push on but the end is in sight and I honestly promise you that it won't seem like it now but it really does come around before you know it