how do I tell my girlfriend that, after all our problems and nitpicking, that I have less good feelings for her? I don't think I can get them back.

You tell her that with all your disagreements,  that it is obvious that you are not meant to be together. It would be best if you broke up and live without the stress of the constant battling. Then you wish her well and tell her you hope she finds someone that will make her happy.

I will simply say that in a long term marriage it is quite normal to fall in love with your spouse over and over again.

But also, love is something that one does---as well as something that frequently causes good feelings.

Couples counseling can give you both necessary and useful skills to make things easier in a relationship.

i think that the best way to say that to her is to take her out, go on a walk with her , have lunch , have dinner , spend the day with her but before that , during lunch time tell her about how youve been feeling and what your decision is, then tell her that you want to spend the day with her, if she says she doesnt, insists a bit and then let go, tell her that shed be fine without you and you guys need space n stuff, but remember that you tell this to her during the day , not at night, this way she will know you have broken but atleast shell be able to spend some more time with you and itd be easy for her to get over it!

Talk directly to her. ask how she feels