It's most likely something to do with technique, a belt won't help that.
With that being said, after addressing the real source of the problem, whatever it may be., I would say definitely get a belt.
Not a fabric tapered belt, those are bulls hit and are not optimal for moving the most weight.
Just to agree with some others, learning how to brace properly is most important, and training beltless certainly has its place.
Belts let you lift more if you are using them correctly, point blank period. So if your intent is to get stronger than get a good belt and learn how to use it properly for your heavier sets.
I have an inzer forever belt, I got the prong one because you don't need a screw driver to adjust tightness, I like varying tightness for different exercise's so versatility is important to me.
Pioneer, as mentioned, makes great belts, I got one for my girlfriend and she loves it, but I know in some federations they are not competition legal so you may want to check on that if you're gonna compete.
At 5'9 I think a 3" belt would be the way to go, I'm 6'3 and I use a 4" belt but I have a really long torso so it doesn't get in the way for any of my movements.
And I would suggest a 13mm one, they are more heavy dude and provide more support. They take longer to break in and are maybe a little less comfy than the 10mm but I think it's a fair trade off.