Hi

You can appear in JEE Mains exam thrice only. So for 2019 JEE Mains candidates who qualify 12th class in 2017/2018 or appearing in 2019 are eligible.
Now that you have qualified your boards in 2018,  you were eligible for JEE Mains 2018, then 2019 and also 2020. SO if you feel that in JEE Mains 2019, you were not able to do well and could not get your desired college, you still will have a chance to appear in JEE Mains 2020.
But remember that will be you last chance. Also you wont be able to appear in JEE advance as for advance, you need to have qualified class 12th in the same year.

Hope this helps
Feel free to revert back incase of any doubt
Thanks!!