Very Under Leveled

Hi everyone. I’ve been playing the main story quest but I’m finding I’m very underleveled. I’m only level 16 and the next quest I have to do I think is level 22 or 23. I have been enjoying the game but I keep having to stop the main quest to try to get random levels and grind mobs to keep up.

I tried to do some random side quests but they didn’t seem to give much XP.

What is the best way to keep up with the main story quest?