Defense Tips

So I'm curious how to handle bigger opponents backing me down into the paint? I'm not a big guy. I am strong and I play tough defense but bigger guys have a tendency to just start smashing into me trying to go closer in the paint and other than pulling the chair out every now and then what am I doing wrong? I hate calling fouls so sometimes people end up knocking me down nobody wants to get beat by the old guy.

For reference I am a 44 year old white guy. I'm only a 140 lb at 5'10. I usually don't have to play huge guys but because I'm an older dude I get some weird matchups at the gym.