Wrestling is a fully body workout, however unless you are serious about it, you'll only end up fucking up your knees and back. I only know a handful of people who are older than 30 who seriously wrestle, but a few of them were in their 50s, 60s, and one even in their 70s. But again, it was an everyday thing for them. One I knew very well was my Romanian coach who wrestled for the USSR before he escaped the iron curtain. He'd punch and kick me, but I learned a great deal from him. Keep at boxing if you want brain damage. I'd suggest Jujitsu instead.
Everyone calling you weak probably can't do much more. The fact you can do 300lbs shows you are doing something at least, which is more than most can say. Keep at it.
All online workout discussions always boil down to the same thing. You'd be 100 times better off spending your time in this thread actually working out. Worst case you just become gay like /fit/.