Success breds success. It is hard but try to ignore, or at least put in compartment for a while, things that bother you and focus on some good points of yours. If too down right now to make much of a list of good poinst then start with things that interest you and begin to do them.
Keep it simple: You want to lose a few pounds and like/want to be fit then start walking/jogging and build it up. As you go further and all that your confidence will build over this positive accomplishment. This example only applicable if you like working out and/or being fit.
I let myself go, just gave up a while ago. I was the champion of being a slug. I remembered how I felt when I used to run and when I was a real tiger in bed (or in my head anyway - smirk, smirk), though nothing special about me I was really fit and woman like many, many things and this is for obvious reasons one of the superficial things they like (yes, they like the other things too, even more). I got a long way to go to have confidence but I went three miles three times last week and I am finding my smile coming back!!!
Just my humble to cents: build it up one step at a time and focus on strengths/like (avoid working on weaknesses, build safeguards for those)