Proper fit is everything when it comes to being comfortable in a wetsuit. It's essential that you try the suit on before you buy it (or, if ordering online, be sure to ask about the seller's exchange policy). A suit that's too loose will allow for water exchange which means it won't keep you as warm as it should. A suit that's too tight will constrict your movement which will cause you to tire quickly and be very uncomfortable.
I hope you washed that thrift store wetsuit well before using it. It's one thing to wear a suit that you've pee'd in. It's quite another thing to wear a suit that someone else has pee'd in...

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