After having an open bite before braces, every night when I put my retainer in, it feels tight. What am I doing wrong?
I am glad you are diligently wearing your retainer at night. This is important to keep the teeth from moving back to where they were before. If you just had your braces off, your teeth may have more of a tendency to try to move back because your bone is still in breakdown-buildup mode. You may just need to wear the retainer for more time during the day for several weeks and then wean yourself to wearing it only at night.
If you had braces removed a long time ago, you may want to try to wear the retainer for more time daily. Over a period of a few weeks you can wean yourself to wear only nightly again to see if this helps.
If this does not help, I would recommend oral myofunctional therapy (OMT). Having an open bite in the front teeth means that your tongue and lips were not coordinated before braces, creating the open space. OMT helps retrain the muscles so that the tongue no longer pushes on the teeth and the lips help hold the teeth inward.
OMT will encourage long-term health of the muscles and encourage the teeth to naturally stay in the space the orthodontist or dentist put them. This will prevent the costly need to re-correct the alignment in the future.