I love you since forever.
In the sentence, the simple present tense has been used with "since". I'd like to know whether it is possible or not.
No, it is not grammatical. This is because the present tense ("I love you") refers to things that are happening now, like reporting a current event or state ("I love this restaurant") or happen in general ("I eat here every week").
So the grammatical way to phrase it is "I have loved you since forever."
But we're not quite done! "Since forever" is a very informal phrasing. You can use it and people will generally understand it to mean "for a very long time", but the more common phrasing would be "I have loved you forever."
No, it's not correct to use since forever in the present simple, though it's understandable. Grammatically, you should say:
I have loved you forever.