The right to sue letter just means the EEOC is not going to represent you and you have now exhausted administrative remedies which allows you to file suit in federal court but you have to do so within 90 days. It is difficult to examine the merits of your case based upon what you've provided so an attorney will need much more information. You will want to request the investigation file from the EEOC investigator which will be extremely helpful if you pursue a federal claim.
If you are interested in pursuing a lawsuit, you must speak to an attorney quickly. Once the 90 days from the letter passes you lose your right to bring a lawsuit.
As to your chances to win, that is not a question that can be answered over on an internet blog. Again, you need to consult with an attorney.
However, if you feel that you have been wronged and you are inclined to bring a lawsuit, do not give up simply because the lawyer for the employer that you are going to sue says you will lose. An EEOC negative declaration is not, all by itself, evidence for the court or grounds for the dismissal of a case.
This response does not create any attorney client relationship. The response is informational only and is not a legal opinion.