In my opinion, if you purchased the game to have fun, but you're not enjoying the fun, perhaps it's either time to step away or re-evaluate your desires within the game.

If you are inclined to continue, then I would say don't get discouraged for a long term. There will be days where you feel you're underperforming and there will be days where you are completely in sync with your kit and the fight and having a blast. The days when you are under performing, look into what you can do and keep improving.

Overall there are a few things you can do.
-Potentially consider changing jobs. I don't play DPS jobs at the highest tiers but my understanding is that SMN is a really complicated class and a lot of nuances to keep track of. This might not be for you so perhaps try something else.
-Pound the crap out of the training dummy until your "rotation" becomes muscle memory. I don't believe SMN really has a rotation but being able to think of your priorities on the fly will help you focus on other things in a fight.
-Do you understand the fights? A lot of optimizations revolves around understanding the fights. Everything is structured and being in position or being aware of where you'll need to be in the upcoming ten seconds will get you thinking about what you need to do to maintain DPS while doing mechanics.

A lot of players say DPS have the easiest time of the three roles and it's not entirely false. Out of all the responsibilities in the vast majority of content, DPS responsibility are light compared to healer and tanks. However, being able to play DPS at the highest tiers of proficiency is ridiculously hard. This is not to be discouraging but you're competing against people who have had years of fights pounded into them. Even though the kits have changed over the expansions, they need less time to adapt to the new style versus you picking it up fresh, considering you've joined the OF as of August 2017. These players also understand every nuance of their DPS kit and have the best gears available to them. It's an uphill battle, especially now that players are getting BiS'd out in week ten of the raid tier.

The fact that you have a drive to improve and want to continue to get better already makes you better than the vast majority of players. If the SMN kit is fun for you, then I say keep going at it and see what you can do. Keep going to improve, pound that dummy, participate in content, get the gear, and try to keep pushing that number upwards.