Most Helpful Guy
I don't think you truly believe he downgraded. Based on my guestimate of his age, he was never ready for "wife material" and is still in the mood to party and drink with friends. You were the anchor to a stable life that he currently does not want - so he dumped you for a big titty party girl and it's a major blow to your ego. Hence you've created this imaginary belief that he downgraded.
One day when he's older, he'll leave his party days behind and search for someone similar to who you are now. There's not much you can do but to move on. Luckily, girls tend to forget about ex-boyfriends more easily. I suspect he will regret his decision 5 or 6 years from now, when he wants to settle down but doesn't have anyone suitable to make his wife.
Most Helpful Girl
What do you do?
Stop tracking the progress (or lack of progress) in his journey and focus on experiencing success and progress in your own. I hate to break it to you, love but what he does with his life is no longer your concern nor your business. If he chooses to stick is dick in every hole in the city, catch gonorrhea, then get married in Vegas the following month, it is no longer your concern for you to think about, worry about, or be emotionally effected by. You two are no longer together so it's time for you to move forward. Don't stay stuck in reverse mentally over some jerk who cheated on you.
What you need to do is focus on how you are going to be the absolute best woman you can possibly be so that you can experience an amazing, fulfilling, wonderful journey in life! :D Now's your chance to really be selfish as a single woman and strictly focus on attaining all of the delightful things you wish to receive from life without the process being disrupted by some sleazy, deceitful asshole's indecision. That's exciting! Now is the time for you to take an honest look at your strengths and shortcomings then formulate a plan to balance them out and strengthen your attributes to their absolute best so that you can be prepared to enjoy doors of opportunity whenever and wherever they open! You may want to start at what seems to be arrogance and an inflated sense of your skill set for a man. I could be wrong, but it's pretty cocky for you to deem yourself as marriage material and demean her as less because she's different from you. Anyhow, stay positive be strong, nurture your self-esteem and sense of self-love, be graciously kind and patient with yourself, and move on so that you can get back to enjoying life!
P. S. He most likely chose her over you because deep down, subconsciously he knows what he deserves: Less. Somewhere on his realm of consciousness, he's aware that he deserves significantly less than what he had so that is what he seeks and approaches without even realizing it.