A good solution for this problem is to make a script file (FILENAME.py) in python that you can run in Abaqus through the File--> Run script option.

You first run your simulation which creates a odb file. You can load this odb file with abaqus and use the python script to get your reaction forces and loop over all nodes and sum them. Below I added a piece of the code I used, it doesn't does axactly what you want, but I assume you can change it according to your needs.

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odb = openOdb(path='MYFILENAME.odb') #if the odb file is in your working directory
numFrame=odb.steps['MYSTEPNAME'].frames[-1] #-1 means last frame
RForce=numFrame.fieldOutputs['RF'] #RF are the reaction forces, you can also change this to U for dispalcement
regS1 = odb.rootAssembly.instances['MYINSTANCENAME'].nodeSets['MYNODESET']

FX = RForce.getSubset(region=regS1).values[0].data[0] #change the .values[0] to .values[1] or higher to change the node
FY= RForce.getSubset(region=regS1).values[0].data[1]

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