I'll try to dish out advice based on my mindset when I play splat zones. Hopefully this can help you out.
When the match is about to start, keep in mind what weapons your enemies have. This can give you an idea of how fast they can claim a Zone. For example, if you are on a team of shooters and the other team is filled with 2 chargers, a dynamo roller and splating, Your team can clam the Zone faster then they can. But the kill power is better then yours since they can pick your team off from a distance. The enemy team will have to focus more on killing so they can claim the zone. And if they can't do that well enough, your team can just rush them down, take them out, get the zone and cover their area with ink. Thus making it really hard for them to get the zone and move. So keeping in mind what weapons the enemy has (and even your teammates) lets you know what you should and should not be doing. If you are on a team with 2 chargers, and a splatling for example, you have to be more aggressive and be in the front lines. If you die, the other support weapons on your team will get rushed down. You really can't sit back alot (This does not mean just go in with no caution though). If your other teammates are good with their weapons, then they will have your back and be annoying to the other team. Slow them down or even kill them. Or you be the distraction and your teammates get a sneak kill since the enemy is worry about you atm.
You don't really wanna go for the zone most of the time at the start. It's best to drop the numbers of the enemy team. If you can get the team down to half while you still have at least 3, you can really cover the zones and not have much to worry. You can cover space quickly and limit the movement options of the enemy as well as the chances of getting sneaked attacked. In some cases, you could go for the zone first but only if your team is fast, otherwise, drop numbers so you can gain the zone. Also when going for the zones, do not go at it alone. It's best to be near an ally so if an inklling was to shot at him, you can take him out or double team them. Going alone usually leads to one getting splatted, then numbers get reduced. if you are good with your shots and have good movement, you could go at it alone and splat some inklings. If you can get 2 kills that is good, even if you die and have at least 3 members. otherwise stay close to an teammate but not so close that if he was to die, you die quickly. Have a bit of distance to cover/double team.
If Both you and your enemy are inking the zone, this could be a chance to get kills. I'll give you an example. I'm a jet squelcher. I'm in a high area trying to gain control of the zone. There is somebody on the bottom doing the same thing. I give up the tug of war and decided to aim for him. Now what happens is I either get the kill, or he runs off. Stopping him from what he is doing. Stopping one from trying to achieve an objective is just as good as trying to gain it yourself. And this is a smart play by me cause as a Jet squelcher, its not that good at covering turf well. So he would have claim more space them me. And I am being still for a while. Giving the enemy a chance to sneak a kill on me. In your case with the weapons you use, you could go around and take them out or just toss a bomb at them and hold ground. OR support an ally lower numbers, then go for the zone. Sometimes tug of wars depending on the weapon are not worth it. Don't focus on the zone (unless you have no choice) and just try to care the enemy off if possible.
With all that said, you state that you gain control but then lose it and get destroyed. I think the main reason for this is when you gain, the zone, you don't push advantage.I'll give an example. Me and my teammates take out the whole enemy team. And gain the zone. After we do, we just stay around the zone. While this is not completely bad, we are letting the team get to the area and then fight them off. This is giving a chance for the enemy to fight back, win, then get zone. You don't wanna do that. If possible, when you gain zone and lower numbers, head straight for their area and limit space. This makes it a pain to even get close to the zone. Which might be happening to you. If you are unsure to either go to their side and flant or not, you can always cover exits to get near the zone with ink. Cover all the paths that an inkling could get to the zone from their sides without having to flant. When you do this always look around the exits you covered. if you see an inkling shooting near it, that means they are trying to get in. Now you know where they are. And can kill them head on or hide around that area and when they get close, pop out and get a quick kill. Take control of your advantage. Push your advantage. Don't always wait for a fight to come to you. Stop a fight from even happening.
Last tip is if you are the only one on your team, do not try to go in getting the zone alone. Hold your ground and wait for the team to come back. Hold your ground or back off a lil. But don't trying to get the zone alone or pick fights. Be defensive till the team is back. Then work on dropping numbers. And claim zone.
Hope all this helps and good luck.