Lubuntu is stuck on loop

I installed Unity using sudo apt install ubuntu-desktop command. It worked at first but when i rebooted the computer, it doesn't give me the option to use Lubuntu Desktop environment, it's just stuck on Unity environment and it doesn't even launch properly, it gets stuck on purple screen when i enter password. Does anyone have any idea to fix that?

Edit: I executed the command "ls -al /usr/share/xsessions/" and here is the list:

Lubuntu.desktop openbox.desktop ubuntu-communitheme-snap.desktop ubuntu.desktop

I also executed the commands that were in the answers but still no change. I included the picture of my screen now. And my Lubuntu is 18.04, modern.

https://i.stack.imgur.com/EXPJm.jpg

Edit 2: Firstly, I learned that I was wrong about having installed Unity but that wasn't the real problem anyway. But I did manage to solve the problem. I somehow chose gdm3 as greeter which doesn't let you choose a different session apparently, or maybe mine was outdated or something. I then re-installed lightdm and chose that as the new greeter during the installation, so it solved not being able to choose a different session problem.

For the stuck on purple screen problem, it was actually not such a problem which made me laugh at myself. It was how Openbox session was supposed to be like, if I right clicked the desktop, I would have known that it wasn't actually stuck on launch. Anyway, the real problem was that the default session couldn't be changed but now it can be and this turned out to be a good lesson for me.

Try this,

Simply start Ubuntu like you normally do. When you see the login screen, don’t sign in. Instead, press Ctrl + Alt + F3 on your keyboard.

NOTE: Ubuntu will drop out of the GUI login screen and into a Terminal mode.

Enter your username in the terminal prompt and enter the password when asked. Now, run the following commands:

sudo apt update
sudo apt-get -y dist-upgrade
sudo apt-get -y autoremove
sudo apt-get -y clean
reboot

After executing the commands you will be able to login into Ubuntu successfully.

NOTE: To exit the Command line, you can use a keyboard shortcut that is Ctrl + Alt + F2, to get back the Graphical User Interface.

You are probably stuck on the "purple screen" because you have partially uninstalled your graphical environment and tried to log in using it anyway.

To repair damage like this, you may need to log in via a text console (Ctrl-Alt-F3 for instance), but getting to a terminal window in the GUI works too.

To make sure lubuntu's graphical environment is installed, run apt install lubuntu-desktop

Once that completes, you can return to the graphical environment (alt right arrow until you get it) or log out if you were already there.

Then, when you are logging in, before you enter your password, click on the icon in the top right corner of the login window, next to the login name. This should give you a menu of graphical environments, which hopefully includes lubuntu.

Once you have selected your desired graphical environment, continue logging in normally.