Should I continue this relationship? Help!!?

You have the complete right to Justin if Lindsay isn't his girlfriend. I understand how this is so hard, as you don't want to hurt your relationship with Lindsay, but frankly, if he kissed YOU, he likes YOU, and if she wanted him, she could have had him before. I would just calmly say sorry to Lindsay for hurting her feelings unintentionally, but as of now, Justin is yours. Good luck! =]