Ubuntu "eats" space on disk

I have a strange problem with Ubuntu. The space od my disk is eaten by system, probably. I have a disk separated into 3 areas - boot, exchange and free space for data. The sizes are 60GB, 16GB and 180GB. I feel the disk do not work fluency. The problem is connected with boot part - Normally I should have there 40GB of free space and after boot computer every day I have it, butafter few hours this space is smaller (currenlty 26GB of free space) and I do not know why. I dont install any programs, I just surf internet or use intellij - thats all. And free space is smaller. This is my output from df -h:

A you can see /dev/sda1 is the problem - it take more space all the time.

I have tried to do all thigs I know: clean, autoclean, clear logs and also used BleachBit - nothing helped. But after restart I have againa 40GB of free space which is reducing by the time I'm using system. What could be a source of this problem and how to repair it?

EDIT:

After installed ncdu, I got this info:

 8,9 GiB [##########] /var                                                                  
    4,2 GiB [####      ] /usr
  793,3 MiB [          ] /lib
  723,3 MiB [          ] /opt
  137,7 MiB [          ] /boot
   14,7 MiB [          ] /etc
   12,2 MiB [          ] /bin
   11,0 MiB [          ] /sbin
   88,0 KiB [          ] /tmp
   56,0 KiB [          ] /snap
   48,0 KiB [          ] /root
e  16,0 KiB [          ] /lost+found

Looks like var folder is too big. Go deeper:

5,8 GiB [##########] /gdm3                                                                 
    2,3 GiB [####      ] /snapd
  323,2 MiB [          ] /mongodb
  191,8 MiB [          ] /apt
   73,5 MiB [          ] /dpkg

Gdm3 is very big and its logs:

/var/lib/gdm3/.local/share/xorg 
                         /..                                                                   
    3,6 GiB [##########]  Xorg.0.log
    2,2 GiB [#####     ]  Xorg.0.log.old

How can I fix it?