Honestly when it comes to break ups my opinion is to work on yourself.
If you have any hope of getting their return it usually lies in fixing the things that went wrong, accepting the past is a done deal, and being willing to get your shit together and not demanding they meet all your whims while refusing to do the same for them.
One of the things that’s not even magical and gets an exes attention is when you are doing great without them. Not trying to get them back, appearing to not want them even if you do. Appear strong, stable and ready to take on this new life without them- even if you don’t feel it yet.
I recommend taking up a new hobby (yes doing magic yourself counts) and getting out of the house and living. Letting this obsession eat you alive will repel the ex and deepen his new relationship.
I don’t practice Demonolatry personally, though I consider myself familiar with the practice. I’ve worked through several of S. Connolly’s books and if I recall correctly she has what she calls Chap books in kindle unlimited and on her website for beginners.
They don’t get too in depth and may leave you with questions, but we could probably help some with that if you read them and had questions still.
Most of her other works are a bit tough for beginners as they require an eye for discernment and the ability to put your own rituals together.
I highly recommend her Book Of Agares though even to beginners- I did it when I was pretty new and it’s basically the creation of a magical journal that serves as a divine extension of your will.
I had really really really high results from it for a beginner, and I couldn’t even procure all of the ingredients and made a substitution or two because of costs at the time. I did use blood in my oil of Agares and I did anoint each page and follow the process of charging it with Agares- I didn’t even have my senses close to what they are now.
I originally was adverse to all the blood letting, now that doesn’t bother me but I still find no need to take a religious or worshipper type approach to my practice though I do offer honor and gratitude in addition to other items such as blood and menses where it’s appropriate for me and my workings.