I got another compliment yesterday. Why don't I believe them?

I've been working out for over 2 and a half years now. I can believe that I objectively look good, but I just don't feel like I really do. It makes no sense, I know, but I can't explain it very well. I was a very very skinny kid, borderline underweight in my late teens and have gotten around 48 lbs since. I still feel extremely skinny and physically unimpressive.

I've recently started getting what I like to think are genuine compliments from people who aren't necessarily as close to me. I obviously realize they'd have no reason to lie, they could just say nothing, but people have started noticing my physique it seems. When they do comment on it, I feel like they're saying it as a joke, even if there's no implication, or as if it is to make me feel better about myself, despite them not knowing that's something I struggle with.

Yesterday, on my way to the gym, I ran into a friend of a friend I hadn't seen in a while and we stopped and had a quick chat. I mentioned going to the gym and he said that you can tell I work out and have gotten quite a bit bigger. I couldn't even say thanks because I guess I don't believe it. Just laughed and said yeah like an idiot.

Why can't I get past this mental block? Why can't I see myself as fit? I've worked very hard and I feel like I deserve to feel good in my own skin and I just don't and it's super frustrating.