What does it mean if I dream about my ex while having a girlfriend?

So I am happy with my girlfriend she's never done anything wrong and she makes me happy. Well last night I dreampt about this girl I use to talk to and in all honestly I view her as the one that got away, it was my fault and I wasn't ready for a relationship at the time so it didn't work out. My mind wonders and from time to time I think about what could have been if I met her later in life. That aside I moved past the part of missing her and moved on, I hadn't rhought about her or even seen her at all until yesterday on Instagram when she posted a picture and I saw her for the first time in about a month. That same night I had a dream where we were kissing and yet she knew I had a gf and still did it anyways. I woke up thinking about her and wondering what could have been. It's been on my mind and I feel bad I even think about my ex while with my girl. I am unsure what this means or what to do. I don't think or dream or miss this girl at all except today and last night. Any advice will help.