Forgiveness and understanding are your friends right now. Embrace them. 🙂
Your girlfriend did not cheat on you. You basically dumped her after 3 years to do some “soul searching” — whatever that means. It doesn’t matter that you seemed to find your answers a week later, and got back together with her again. [i]During that one week, she was single[/i][i] — because you made her single.[/i] You are just as responsible as she is for the situation you’ve put yourself in.
That she lied to you about sleeping with someone during the week you were broken up is really just a white lie. The reason I forgive this lie is because she doesn’t lie to you normally. There’s no evidence she’s lied to you before. And most importantly, it was none of your business if she slept with someone during that week. You had no right to ask her whether she did or not, and she had no obligation to tell you.
If what you’re looking for now is an excuse not to propose to her after 6 years together, there’s nothing I can say to stop you. I would never advise any woman to stay with a guy for as long as she has without a ring and a wedding date! So consider yourself fortunate and blessed that you still have her.
My advice to you is to man up and understand the position you put her in, and offer your apology for that, and your forgiveness for her white lie to you.