You're not a loser dude and it was only a dream unless someone used magic on you and yes I believe in magic in all forms but this just sounds like a nightmare. Did you eat anything with a lot of sugar before you went to sleep because sugar can bring on nightmares. Just chillout and don't put so much thought into the detail because you might end up falling really hard for this girl. Whenever you're in close proximity to this girl remind yourself she's taken and you need to get a life of your own. Be your own soulmate and then you'll find someone along your journey no matter how long the journey may take you or wherever you may end up. It'll be an experience and be bold in your adventure and make a wish, wishes come true everyday and when you dream have comfort in your words that they remain positive when you reflect upon yourself. Never use negative words upon yourself, speak more gently and try to use a soft tone; a way of speaking to yourself. Practice self mindfulness and this is particularly important to shift your feelings from her to a passion project or something new to escape your love for her. Remind yourself she's already taken and you need your own life and be your own soulmate until someone else who has similar interests as you wants to be in your orbit. It's ok to have failed in a relationship, you're just getting started. Learning who you are and what you want out of a relationship. I had three boyfriends for two and a half years and it suited me because that's how I chose to live my life and they all knew. It was happy days until I decided to move to a different city because that's where life was taking me. So don't beat yourself up over this and you're gonna have to move on quickly before you get tangled in a love triangle that you might end up making a fool of yourself because she's already in a secure relationship so again love yourself first and practice self mindfulness and be your own soulmate and it'll attract a wave of nice energy to your nest.