You need to apologize and—more importantly—demonstrate that you can be trusted. This happens a lot. Did it screw up your chances of winning her back? You're not supposed to care that much, remember? Stop when you've fallen into a routine.
At the time, all you can think of is her body and how it feels and is reacting to your touch.
What Does It Mean When You Hook Up and Have Sex With Your Ex Girlfriend
Don't hang out with your ex in front of your friends, go to a party with her because she doesn't want to show up alone, or tell your friends that you've been hooking up with her again. You need to apologize and—more importantly—demonstrate that you can be trusted. And you miss them. So if you find yourself in bed with your ex girlfriend a whole lot sooner than you ever thought possible, just know that there may be deep psychological reasons acting on you both to try to extinguish the angry feelings. Sex with the ex can create a new bond after cheating, but be prepared: Later I will also get into the consequences of hooking up with your ex so quickly after the breakup. Let her know it's time to move on.