Can I choose to end shabbos earlier? Is it considered a neder if I extended shabbos longer, kept Rabainu Tam for more than 3 consecutive Shabboses. I kept it to add to my shabbos, I never verbalized that I am making a neder.
If you kept it for more than 3 Shabbosos then even if you didn’t say that you want to continue it would still become a neder, and it would need hatoras nedarim. If, however you were moser moda’ah, then that would help that you don’t need to be matir neder.
As a side point it is always good to get into the habit that whenever you do a good thing to say that it is bli neder.
Y:D 214-1, Shut Chasam Sofer Y:D 231, Meishiv Davar 2-73, Kitzur Shulchan Aruch 77-4, 7, Shulchan Aruch Horav 248-13, Salmas Chaim 467 (new edition Y:D 62), Kol Nidrei 73-8.