Do I need to buy lube?

I'm only 21, but I feel like I'm not producing as much natural lubricant anymore despite having lots of foreplay.

Yes I would get wet during foreplay, but then once we start having sex it's like I'm not constantly producing enough to keep it wet. It's like it dries out.

I normally don't really realise it until after we finish having sex my vagina opening is kinda painful. Most likely from the rough friction.

Do you think I need to buy lube? Or is there any tip so that I keep producing enough lube naturally?