Knowing your character build and party composition would make it easier to offer specifics. Here is my cleric/tank checklist with a few assumptions about your character and party:
-The party has strikers (e.g. rogue, sharpshooter, sorcerer) and your goal is to force enemies to waste time on low-odds attacks on you before they can attack the strikers. To be a tank you want high AC, good hp, and a way to harass enemies that move past you towards the strikers. If your actual goal is to increase your overall melee damage, then ignore everything I'm about to say.
-Stats at level 6 are something like 16/10/14/8/18/8. You're strong, wear heavy armor, and have athletics skill proficiency so you aren't easy to shove or grapple. Spiritual weapon/spirit guardians will be at +7/DC 15.
-Improved concentration checks: started at ftr 1 for proficiency in con saves, or started as a variant human cleric 1 for either the war caster or resilient (con) feat at level 1. At level 6 your Con save is at least +5 or you have the War Caster feat.
-At ftr 1 you chose the Defense FS for +1 AC. Assuming you have upgraded your armor, use a shield, and/or have a magic item that improved your AC, your AC is at least 20-21.
-In combat you are using spirit guardians/spiritual weapon/dodge like OverlordOcelot described. IME casting spirit guardians will cause you to be targeted by a lot of attacks, which is the main goal of the tank, until the enemies can shut it down with a failed concentration check (that's why you need to improve concentration before cleric 5). Unless you are outdoors and enemies can move anywhere they want, they can't just ignore you and run through your spirit guardians because you'll move faster and follow them and put them back in range. The slowed movement will buy your strikers an extra round or two of attacks.
I don't know if this is your first 5e cleric or not, but maybe what you are experiencing with this character is the built-in diminishing of the cleric's relative melee power as you shift from Tier 1 to Tier 2. At Tier 1, with your high AC and martial weapons you feel as good as any fighter in melee, and better even when you have your spiritual weapon up. When my cleric got to level 5 and we were fighting giants and demons and such, my single warhammer attack all of a sudden was lame compared to rogues with their +3d6 sneak attack, warlocks with their 2(d10+d6+4) hexed eldritch blasts, or anyone with extra attack. I went from being a front-line fighter to almost never taking the attack action. I still was up front blocking and soaking up attacks, but at levels 5-7 I did a lot of spirit guardians/spiritual weapon/dodge or just buffed the good strikers and used the bless/sanctuary/dodge combo. After I got my damage upgrade at 8 I started doing more melee attacks again.
I would stick with Cleric. The next 3 levels are all very good cleric levels. At 6 your channel divinity will heal up to 60 hp per short rest without using spell slots. At 7 you get 4th level spells including Death Ward (very tanky!) and Banishment (an encounter shutdown spell), and at 8 you get a melee damage boost and an ASI. Taking more levels of fighter would of course make you a better fighter, but never as good a fighter as if you had just been one all along. Nor will you be as good a cleric as you would be if you had just taken more cleric levels from here on out. If you eventually got to Tier 3 you might wish you had 3 attacks and Indomitable, or find out that you don't have the high level cleric spells you need to support and protect the party.
