Maternity Leave

Hi everyone, I'm new to the boards and have been lurking for a few weeks.  I'm having twins due at the end of April, and I would say I'm having a pretty good pregnancy so far (though I have nothing to compare it to since this is my first one).  In the last few weeks though some issues have come up that have made work much harder - specifically a right thigh numbness since November that has progressed to a numbness + pain when standing for longer than 5 minutes (not an exaggeration).  I've been told by my doctor that it's normal and probably just a pinched nerve, and I was given an ultrasound to check for blood clots just in case (negative!).  The problem is that I'm a teacher, so staying off of my feet is not always easy, so I'm in pain most of the day even when wearing the compression stocking they want me to wear.  Recently I've also had a lot of dizziness (which I've also read is normal because of all the re-directed blood to the uterus) but again, it makes it hard when I'm standing all day and when things like trying to drink enough water without the luxury of being able to go to the bathroom when I want or walking up the stairs being exhausting is just making things really hard at work.  I originally was going to start maternity leave at the end of February (about 8 weeks before my due date) but today my husband and I talked and if my doctor will approve it, I am probably going to go out at the end of this month.  I guess I'm writing because I feel guilty about going out earlier than I thought.  I always imagined that I'd be super mom and work up until my due date but my husband keeps telling me that this is a different situation and it's okay.  It just makes me feel like a failure.  Maybe I'm overreacting and this isn't a big deal, but has anyone else felt like this?  Did anyone have to stop working earlier than you expected?  How did you deal with it?  Thanks for your responses in advance everyone!