You were feeling good yesterday, because you were with him....but today, you know he's with somebody else....this is an attachment...the outsource that controls you to feel happy or sad....
Just try not to think about him....or if he comes to your mind, just let it go...direct your thoughts to something else....then this will be a regular practice for you and your brain will get trained and you will learn to not think about him and happiness that comes with attachments....because that's not a true happiness....

take care
Marjan