I love him but the ex is coming between us.

okay so.i am in relationship wid dis guy he is 6 years older dan me nd m 17 ryt now…i do luv him alot..we had brkup once wen i got to noe dat he was cheating on me…though he said he was serious for first 3 mnths(nd i feel dat it is true)…v had no contact for 3 mmnths after brkup nd i stil had hope dat ve wil b 2gether again…
d thing is d gul he was cheating me with was his first luv nd he was serious 4 her bt she came out to be a bitch….
now ve r 2ther again….i love him lyk nything but d gul is cming between us again..she is threatning me to tell my parents nd cols him contnsly(he dsnt answr her cols nw)….i am damn serious for my guy nd i want to be wid him always….but i doubt she wil ruin our relation again wat shld i do???