Well, I could give advice right away but I think the most important thing is to analyze what your problem is. If you rank down that much it means there is at least one aspect in the game you're not getting right. It could be in the skills part (aiming, moving,...) or in strategy/decisions part. Or both I don't know.
So the important thing is to find what your main problem is, and once you know (hopefully you don't get it wrong), you can work on it.
From what you said it seems you decided to dedicate yourself to play the objective. So, for the advice on this role...
- Don't take objective at any moment. Wait for the right time, when you have strategical advantage (at least two more squids on field, you and some mates have specials ready, the overall positioning is especially good,…).
- If the situation is neutral, be patient. The weapons you use are not of the power type. You must build your special, assist your mates, stay quite safe while not being too passive. A power-type would try to make things move. Don’t forget to look at your mates too… Because sometimes your mates are even less of a power-type than you.
- If the situation is especially bad, survive. If you can, find a spot allowing your mates to jump, possibly asking them to. If you go for a kill without dying, remember it will give away your hiding place: sometimes it’s ok, sometimes it’s the worst choice. Adapt to the situation: sometimes, exchanging with the rainmaker is all you need. Sometimes you’d better let them take the lead so that you can finally make 2-3 kills without dying (especially with a power weapon or special though).
And well, you said your mates don’t take the objective; if you specialize in it I would say that’s what you want, mates that kills while you take care of the objective.
As a final advice, let’s talk stuff. As an objective-oriented player I would definitely consider some respawn, especially in RM and TC. Obviously, excepted in some rare situations where you can play objective while being safe, you will be especially exposed. The tower moves? You’re on it. The RM moves? You’re it. Some guy is preventing them to contest the zone? You’re it again. They want to and they will kill you. So respawn does come to mind.
In RM, consider swim speed or/and run speed depending of the rotation. In all modes and depending of your special and rotation, consider special-oriented bonuses.
Finally, you can try to find videos of strong players playing with your weapons. You can find the Japanese names on wiki sites, which could help. That might give you ideas.