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Well, if they have decided to cheat on you then they will find ways to cheat. It won't be possible to stop them. However yes you can make it harder for them to think about cheating on you by doing certain things, but again there is no assurance. You can only try, that's all.
Some of them are:
1. Always talk to them, give them your time. Prioritise them. That way they will know that they have your attention and that they wanted by you. You know it's always good to feel wanted. Hence make them feel wanted.
2. If they have sexual desires, physical desires, make sure you satisfy that. Not everyday but do so very regularly, that way they will never have an excuse to satisfy their desires elsewhere.
3. Not only sexual desires, show that you love them, always be romantic with them. Have non-sexual cuddles, kisses and show physical affection towards them as much as possible.
4. Never try to flirt with other men either in front of them, or by text or behind their back. Don't let them doubt that you are doing that. You should have your eyes only for them. You should make them feel special and as if he is only man in the world and he is whom you want.
5. Most important point, never try to make them feel insecure or jealous on purpose by any means whatsoever. If you do that then it might give them an excuse to cheat on you.
These are some points which if you follow will make it much harder for them to even think about cheating on you. However as I said earlier, devotion, loyalty are not something that can be imposed or forced on someone. It has to come from within, it comes from a person's own principles, morals, values.
Hence despite you doing all this, they might still cheat.
I mean shit, why do PEOPLE cheat in general.
Men cheat, women cheat.
You even said he cheated on you with your best friend.2