Mostly the opposite perception for me right now and it has changed.
For most of my life, down was small and the mirror looked normal.
One or two years ago, down suddenly became large (long mostly) while the mirror became thick but short. My frame is on the smaller side so my penis should look rather large (length and girth) by comparison in the mirror. Looking down became girthy last September but has appeared longer lately.
You need to drill the facts into your mind AND accept/like/enjoy/appreciate your body the way it is. Perception is malleable and not typically based in fact. Work on moving your self image away subjective perception to facts.
The average penis is about 5.17" BPEL and 4.9" MEG. Check out the calculators like http://howlongismyschlong.com/ and calcSD - Penis Size Percentile Calculator. Pay attention to your volume/overall percentile.
Other than hating circumcision, I guess I have had a good attitude about my penis my whole life and never really felt small or inadequate. I started and continue PE because I wanted to be larger not because I felt inferior.
Get it into your head that your dick is not small or even short and that a lot of men would kill to have your girth. While it is good to work for improvement in life, it is important to start from a healthy mindset.
Also, if it’s for women, forget it. Most don’t care and the ones who are worthwhile are more interested in the relationship. Women will say you’re big when they want to get something out of you and tell you you’re small when they’re angry.
Check out Thoughtful Gold’s posts on perception and female nonsense.