Firstly: welcome to the community, i hope we can help you 
As i read, in general demons work tend to be temporary (although I’ve real that beleth can get a long lasting relationship and fidelity).
In any case you can get help from them to get her back and just then using something to a long lasting relationship.
Demons i know are used in general:
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Dantalioncan: change the mind of the target
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Sallos: can cause love and fidelity in target
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Amon: can cause love
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Vine: can control the feelings of the targer (similar to Dantalion).
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Gremory: can cause love. As i know, this one is useful when the target is in other relationship.
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Eligos: blind affection (but I’m not sure if it’s romantic love)
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Vual: reignite love in a relationship and give you confidence.
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Gaap: to start or break up a relationship.
*Furfur: can cause love (I’ve read that tends to destroy electronics so i would try to keep them shouted down if you work with this demon).
There’s others but are more focused on lust (Sitri and Zepar).
This post have some experience with different love demons: https://forum.becomealivinggod.com/t/whats-the-difference-in-how-different-love-demons-work/64159
About angels i know nothing but I’ve read they work slowly but on a more long lasting way.
Other option is planetary magick to a long lasting love: Venus would be the option on this case
In general love rituals are made on Friday during venus hour to improve the effectiveness.
If you’re already made works with some spirits maybe is your lust for results the reason you’re not getting what you’re asking for. When you perform magick you have to “let it go”, otherwise you can make it fail.