A hassle every week

I have been getting my kids 50/50 for the past year and a half and there has yet to be a week where I'm not given a hard time about picking them up from their Mother.  It's always something.  She has the girls text me that they don't want to come over and that when they are able, they will go to live with her all of the time.  She is teaching them the same behavior that she has and that is one of being a bully and a manipulator. 

She told them that the parenting plan we signed was only temporary and was a trial to see if they would like it.  I found out about this from the girls and informed them that the plan wasnt temporary and that both their mother and myself signed it.  In the plan it calls that we both share custody equally with me paying child support as if they were only coming over every other weekend.  I did this to ensure that their homelife at their mother's was not upset too bad.  This was a mistake as she has done nothing but harrass me reagrding the time that I spend with them. 

She has scheduled them for several activities apiece for after school and while she doesnt take them to these events, her mother does.  However, I dont have that luxury.  It's me and Heather and even if I had that kind of help I dont think I would have them doing that many things.  I enjoy the time with my girls and I don't consider taking them to everything is spending quality time.  

I take them to everything I can because that's what I agreed to in the PP.  To see my girls 50/50 I had to do a lot.  The texts started this past weekend with Olivia and Sophia both texting me and leaving me messages that they didnt want to come over because they wanted to study with Papa Bob (their grandfather) and Sophia wanted to work on a project at home.  As a Father, studying with my kids is MY job.  Helping out with projects is also MY job.  I told Susan in an email, no.  Of course as a bully and manipulator, being told no is unacceptable.  So she replies that the girls dont want to to come to my house and that she will be at the school to pick them up.  Always looking for a fight, she continues to email me the same basic email over and over.  Not picking up what she puts down, I ignore her and set out to get the kids this afternoon.  I show up at the school and ask for Sophia.  The teacher tells me that her grandmother has just picked her up.  I nod my head and drive off to the Police station where I filed a report for parental interference. 

I'm just wondering how much more of this I can take before I snap.  I love my kids more than anything in the world and I hate that I married a monster, but having Sophia and Olivia is worth it.  I'll head out to the school again tomorrow and see if I can get them without  a hassle, but I doubt it.