If you feel like you are too far back you may want to try a lower bend bar. I'm 6' but I run the Carmichael bend which is a low bend. This puts me over the front of the bike a bit more. A Windham bend is a taller bar.
I would start by setting your sag. I know it doesn’t have anything to do with your handle bars but if the previous owner was smaller than you or you are just a big dude, your bike may be compressing too much in the rear making it feel like your falling off the back.
Secondly I would adjust the bars you have too see if you can get what you want. Your bike may have handle bar clamps that you can turn the other direction and move your bars forward or reward depending on where they were in the first place. If not, you can buy them cheaper than a set of bars and grips. Also ride your buddy’s bikes and ask what bend they ride.
They also make adjustable (or just lower) foot pegs that a lot of taller riders prefer. If you’re worried about your legs being too short, you can cut you seat foam down.
If you feel like you don't have the strength to turn the bars, measure the length end to end. The previous owner may have cut them down a bit and will cause a loss of leverage.
If you feel you are slipping off the bike you can add grip tape to your frame to help hold on with your legs. Also an aftermarket seat cover may offer more grip.
Last but not least, you are new so you may just need a little more time to develop an attach position. Remember to keep your elbows up. Knees and elbows bent (not locked). Keep your head up meaning look as far down the trail or track as you can. Hold onto the bike with your legs. I've been riding dirt for 20 years and still have to remind myself of these things… especially when I'm tired.
Good luck and have fun! Welcome to the addiction!
2005 ZX6R (and it's ugly)
2006 CRF 450
1993 CR 500, the big 2 smoke
I can't spell and neither can my iPhone