Feeling restrained by your canvas is a legitimate thing, but it's not one I'm sure I have tips for.
As for struggling to move forward and incorporate details, that's probably the result of not having a goal set for yourself. "Draw the thing" is a bit of a broad concept, so it might be good to switch it up and narrow it down.
Are you happy with the proportions? If not, spend a week or so focusing on proportions.
Are you happy with the motion and fluidity of your getures? If not, spend a week or so focusing on drawing short 30s gestures in as few lines as possible.
Do you feel capable of drawing faces? (I don't actually see any face details in your gestures, but one guy's underwear is surprisingly detailed.) If not, spend some time doing face gestures instead.
Drawing without a goal is not going to improve your skillset, it typically just stalls you and becomes demotivating. There are some things you can work on, but how you work on them depends on your goal. Pick one, and you'll have a much easier time moving forward!
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