So believe it or not you've become chemically dependent on her. It's going to take some time to readjust brain chemistry (quite a bit actually). The catch is that to regain your self worth some introspection is required - but by reliving the relationship you are dwelling on the past and continuing the embedding in the brain so don’t over do it. If you find yourself imagining scenarios where you meet her again and she wants you back because you are rich/famous, or you’re now the hero of the planet with you new hot girlfriend on your arm and you happen to bump into your ex - your doing it wrong. You shouldn’t have to be anything but yourself.
Don't take this the wrong way but you got yourself into this mess by not respecting yourself in the first place - this is common and happens to most humans daily. We all have varying tendencies to want to be approved of by our significant others but understanding why is key to becoming stronger. Don’t be too hard on yourself, again remember that each person you meet and talk with daily is just another fool if you knew their history. And try and pity your ex since either something really messed up happened to make her this way, or, she lacks her own skills and is just as vulnerable to not know any other way to make it through life.
By taking responsibility without beating yourself up for being anything but human you can save a lot of time and move on to a much better future. This is the part in life where the screen reads “congratulations you have made it to the next level”.
Confidence in yourself - there are a few good hacks for this;
Exercise. It can be running, it can be the gym, or it can be intense sport (or even sport that might involve intense concentration compared to less physical). Weight training, even if mild right now, is highly advised. Not just for the physical aspect but for the gym environment and vanity factor / body image / confidence of others around you rubbing off. Healthy diet and in bed before 10:00pm is also critical for making this work & a good chemical balance.
Mental training games. Any game other than video / fantasy games where you improve steadily can increase self confidence. So those seemingly lame online or smartphone / tablet games that require concentration & memory like connecting paths, remembering sequences, adapting to problem solving, even crossword puzzles etc. create new neural pathways.
Sex. This can be a double edged sword. If you’re the type of guy who can handle a fuck buddy without becoming attached and derive what you need from the sex only - great. Just make sure you are not doing anything similar that your ex did to you to this other girl. i.e. she has to be into it only for the sex too.
There’s too much to put in one post but this should give you a good start.
Oh and “Congratulations - you have made it to the next level”.