Starting FB Fit again + new fears about clothes

So, I while ago I wrote about my story and how far I've come and how happy I am. I'm still happy and I'm still working out, so I decided to start FB Fit once again. As I told in my story, FB Fit Round 1 was the first program I ever bought and that I ever tried, it kicked my butt and was never able to finished it even though I started a few times, because I lacked motivation and I was a professional couch-potato.

I'm happy to announce I just finished the second week of Fit, I obviously have more weeks ahead BUT the milestone I'm celebrating is that I manage to do all the high impact workouts instead of the low impact (to those who have tried this program, the first two weeks have a "beginner workout" in case you can't do for any reason the high impact version). This time I took before photos so hopefully in a month a half I can come back with good news.

Anyway, aside from that, since I started this journey I lost a lot of weight and some inches so the last few days I've been going shopping to find clothes that fit me. It's so weird finding clothes that actually fit me lol Since I always wanted to hide behind my clothes all of my pants and shirts were usually bigger than it needed to be, but here comes my new fear. I went to American Eagle and bought a stretchy jean size 8, it didn't fit me correctly so I went ahead and changed it for a 6... That one fits me just right, but I can't help but think the "what would happen if I gain a little bit of weight? What if I feel off the wagon or something happens with my diet and these pants won't ever fit me again?", I bought an XS Shirt the other day (the style is bigger than usual shirts so...) but I returned it and exchanged it for the S cause it was a bit looser and because I thought of the same fear of the what would happen if I gained a little bit of weight again for x or y reason?

How do you manage to fight this fear if you ever had it or how did you start to "learn" how to shop again for yourself? I'd really appreciate some advice cause I think retailers are starting to get annoyed that I go so many times to change the same clothes just with a different size lol