This is a simple question of copyright. You have written some code - you can use that code however you wish, and you decided to contribute that to a GPL library. That doesn't change any of your rights to that specific bit of code, which you can still continue to use as you wish.
However, the same applies to other people as well - they have written code, and the only right you have to use their code is under the terms by which it is made available to you - or in this case, the GPL. "Contributing" to the library does not (cannot) change the terms under which other people contributed to that library. To take this to extremes, let's reverse the situation:
- You have written millions of lines of code in a library, and kindly decide to release it under the GPL.
- You accept a one line pull request from me to fix a minor typo in the comment.
Now I've "contributed" to the library, would you be happy for me to take your millions of lines of code and use them without having to comply with the terms of the GPL? No? Well, that's exactly what you're asking for here...
So to answer your specific question:
Do I have to make my entire project open source
Assuming you are using some code from the library other than the code you wrote, you are using code for which your only right to use them is under the GPL, you must comply with the terms of the GPL, which in non-technical terms means making the full source code available.
If you are using only code which you contributed to the library, then you still own the copyright to that code and can use it however you wish.