Tough decisions

What do you do when you stuck between the rock and the hard place?
You have jumped into the relationship too fast, opened your heart and gave yourself to a person completely, without really thinking about how deep they are into the relationship.

suddenly things take a different turn and you feel trapped, because you fear if you ended the affair the other party will use what they know about you and even blackmail you. You stay in that relationship because of that primary fear about your personal affairs, the other party realises that; and starts mistreating you and knowingly making you feel that you have no way out, so they take advantage of your vulnurability to them, uses you, abuses you publicly humiliates you and make you feel unworthy.

What do you do to get rid of this person and still keep your dignity? This is a dillema that has been brought to my attention, and I do not know to best handle this particular issue. Can anybody please contribute to this? I beg sensitivity to this issue and only serious and genuine responses are welcome. Thank you