How to recover security amount if landlord is not returning it

Respected Sir/Madam, After numerous appeals and negotiations with the landlady, after trying all the respectful and lawful approach; informing all the conditions and situations. My landlady refuses to refund the security deposit of Rs 18000/- despite of his own verbal oath. I am here in great need of help to fight for my heard earned money which she took by the name of security deposit agreed to return which she denies when it comes to reality. I hereby writing this letter with huge hope and anticipations; So hereby I am disclosing all the details/facts/conditions for the meaningful judgment and lawful decision. I opted the flat on 1st floor of in East Delhi -110096 area of one Landlady at Rupees 10500/- per month which is inclusive of water but exclusive of Electricity bill. This flat was 1 BKH out of which I was given 1 Hall, kitchen, attached toilet and balcony covered as room in the month of November 2012. The other room (Bedroom) was rented to some other person. We had an agreement for the same in the Year of 2013-14 and 2014-15. In that time I paid Rs. 18000/- as Security deposit and Rs. 9000/- as brokerage charges to the broker. Prior 2 months, in January 2016 my landlady asks me to take the complete set of flat as a whole 1 BHK with more rent. We talked on it but I don’t require that much space with extra amount as total rent of Rs. 16000/-. So I refused the offer and lives in the same area. But I was in situation that any time I may vacate the flat due to these unavoidable circumstances. At the end of March 2016 I find option for other flat so I directly talk the same to my landlady and told her the fact. In our discussion I ask her to vacate the flat on 14th April 2016. At that time she was agreed that I may vacate the flat and she won’t charge for extra 13 days for that month as she had a new tenant wanting the same flat at the same time. While in our discussion I ask her to return the security deposit and she request me for 1 month to wait as she was not having the money at that time due to her financial situation. (Salary not received for last 3 months – She was State Govt. Employee). So I ask her that in next month (May 2016) she may return my security deposit. After 2 months I called her and ask to return the security deposit, during the discussion she told me that in the month of April 2015 last year I didn’t paid her the rent. So I cross checked all the records and called her again that its already given, after long discussion on it she asks me to meet on Saturday at the home of broker who lives in the same premises through whom I had that flat. During our discussion she told that she will pay only Rs. 2950/- (deduction of Rent for April 2015 as Rs. 10500 and Rs. 4500/- rent for 13 days). At the beginning I bear the expenses for the minor renovation and changes and I told her all these things as she was aware of it but still she was not in mood to return my money. I was so shocked after hearing this and I denied it for what she was saying to me. As she had not any objection while I was vacating the flat and no issue for any other things too. These all things happened in the presence of the broker who was present there at that time, he also knows that the rent is always paid in advance during the tenancy, So I told the landlady again that I had paid rent in cash for the month of April 2015 (for all other 39 months through NEFT) and ask to return my money. The discussion was not complete as she was busy with talking to other tenant for renting her flat at the same time. She ask me that we will discuss it later. But later she didn’t responded on call so messaged her 2-3 times asking about the money. At the 3rd time she totally refuse via reply message to refund the security deposit amount and gives another excuses which is so wrongful, injustice and intolerable and ask me that I owe money to her. I was again shock to read the responses by her and I was so disappointed by such a response from unexpected person who seems to be polite and so gentle once. I called her 5-6 timed but till dated she never responded me properly and ask me not to call again. It’s not only about the money that I am complaining actively but it’s about the hundreds of such tenants who suffered and who have been suffering and for how long they have to; and moreover its question on the moral and principle. Is it the legalized benefit being provided by the constitution? Is there no protective measure for such unlawful practitioners against lawful resident? So I request your kindness for a genuine and meaningful justice/Judgment.