Hi Stars~

Although it has been said already, I have to also emphasize the need for you to seek support...whether it be through school, private counsel, or preferably, from your parents, as a start. (And though their actions may say otherwise, your parents do love you).

From the sounds of it, Matt has a thing for controlling you. The reasons for the change in his behavoir over this time is not certain, nor really is a concern, especially for you. YOU are your concern. Period.

The fact that Matt would acutally lower himself to the point of blackmailing you in order to keep you in his check is enough to make me want me to slap him...Sorry...."been there, done that" sorta of thing...(I know the kind far too well, and NONE are the kind who provide a favorable future with).
Get away from him.

Call his bluff. Take away what power he 'thinks' he has over you by approaching your parents first and confiding in them your worries and concerns.
Surely, your parents know that you are dating Matt. It's kind of hard to hide a relationship that has lasted as long as a little over a year and a half, especially from your parents.

Whether or not your parents want to admit it or not, they are fully aware that you are growing older, (dating), thus they are fully aware that because of the extent of your relationship with Matt, that there is always the probability that your relationship is sexually active.
Your parents may be wearing their blinders to these realities regarding you, but that does not mean that they are not aware of them.
And whether or not they want to be approached, BY taking the initiative and approaching them you are in fact showing that you are taking responsibility of your own self...a behavior which states that you are, in fact, maturing....and deserves respect for.
They may not like it, but they can't help but to respect it.

If you honestly feel that going to your parents is not a possibility, then your next move is to seek support through your school, either with a counselor or a trusted teacher.

Matt is a vexation to your spirit. He will feed off of you for as long as you allow him to.
His behavior has all the classic signs of a control freak...(at least, all the ones I've had in my past. Granted, that was many hundreds of years ago...lol).
But the thing about traits is, whether good or bad, the traits themselves don't change, it's the people and their choices of which they carry that change.

Please, seek support for yourself. You need as much positive influence and reinforcements as you can get for yourself at this time.

Keep us informed. We all care are here for you.

Shangrala