It's fake! She doesn't love me for real, she wants Tara back! That's what you keep having to remind yourself everytime when you and Amber act affectionate.
You agreed to help her get Tara back and as a result, you ended up being Amber's girlfriend,* fake girlfriend in reality. Everyone pretty much believed it when you two came out as a couple, not so long after Tara and Amber's break-up. You promised to Amber that it's just a game, a good plan to make Tara feel horrible and there won't be any attached strings--but you can't help but feel yourself falling in love with Amber.
So what if all these kisses are fake? What if your touch and affection don't mean anything more to Amber? You still wouldn't want to lose it. It's the closest you'll ever get to being with her.
"I talked with Mindy today, she overheard something that Tara regrets breaking up with me and she's jealous of us." Amber says. Her satisfied smirk tells you everything that your plan is working.