I'm going to repeat what others have said here a little bit but I'll try and explain it as well as I can. It's absolutely possible to manifest an ex back but it's all about what you believe in. There's nothing stopping you manifesting the relationship with the specific person, it's just about how much resistance you hold. When it comes to manifesting relationships with specific people we all label it as a big desire when in reality it's no different size or difficulty wise to manifesting let's say a cup of coffee. It's all about the labels and limits we place on ourselves. 

I brought one ex back into my life after a year of no contact and this only happened after I forgave us both for everything that went wrong. And another ex only took a month of no contact and that happened after I decided I was a-ok without him. 

The main thing is being happy in yourself and loving yourself unconditionally because you're the most important thing is you being happy with yourself.