There's nothing wrong feeling that way. If you want her, you have to stop seeing yourself as a loser, and more as someone who can get her. "Whether you think you're wrong or you think you're right, you're right (or something like that). Go out there, forget about your doubts, and before your brain can say no, impulsively say hi. Awkward? Just laugh it off (she'll laugh too). Breathless? Just say what first comes to mind (but still use your brain). Tell yourself that it doesn't matter whether you get her or not, and it's worth a shot. If you don't get her, there's other girls in this world. Try to appear as confident and make eye contact (it's worth a shot, especially because staring at her eyes is a lot better than staring at her you-know-whats ). Don't get meltdowns in front of her. Ask her out only after you know she likes you back. And don't continue to pursue her if she rejects you. Being an aspie might make flirting less natural, but you won't die alone. Want more tips? Look on youtube (the guys in Simple Pickup are really good.)
As for the muscle thing, being muscular doesn't hurt, but you don't need muscles. Just convey confidence with your mannerisms. Also, playing guitar is considered hot by many.
In any case, good luck!
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Your neurodiverse (Aspie) score: 91 of 200
Your neurotypical (non-autistic) score: 122 of 200
You seem to have both neurodiverse and neurotypical traits