Repairing my Credit Score

Did you check your credit report recently? If no, then pull your credit report and check thoroughly.

If you find any error, then dispute it. Also be regular on your bill payments. Pay the bills in full and within time.

Also staying within your credit limit is necessary. Crossing the credit limit can drop your score.

While granting your request for a new credit card, the issuers often checks your credit history, which is regarded as hard inquiry. It can drop your score to some extent. However, there may be other reasons, too.
So, it's better you check your credit reports and dispute errors, if any.
If it's for the hard inquiry, with time, it'll be ok. Just keep adding positive items to your report, such as paying the bills on time, staying within 30% of your credit limit, and so on.

You need to check your credit report asap. Dispute errors if you can find any.
Sometimes opening too much credit account may also hurt your score. It is because multiple hard inquiries also add up when you shop for credit cards in different credit card providers.
You should concentrate on paying off your bills on time. If possible use credit as low as possible.

Use the following sample letters because you will need them now:

https://commun ity.debtcc.com/letters/sample13.html
https://commun ity.debtcc.com/letters/sample26.html
https://commun ity.debtcc.com/letters/sample16.html