In my opinion, I think it was my squishy teammates splitting up and going ham all of the time . . .
They would get knocked out like a toddler with a suicide wish and then I would have to bail them out which lead to me being focused on by all of the enemies . . .
Soon, the group just sort of expected me to charge out on my own and take all of the hits cause they learned that it is not good to get hit by stuff . . .
With my ac of 19, the DM focused on using high dex enemies as well as casters and did his best to make me do dex saves even in the place of spells that required con saves . . .
As the Dm found it difficult to balance the group (since he mostly did guess work rather than working out the difficulty ratings), he decided that he would give people "custom loot" to balance the team and because he didn't see me as being weak, he gave me a shield (+2ac) with lightning resistance and that was kind of it . . .
No enemy ever cast or dealt Lightning on me but instead resorted to poison damage that would give me mental problems? . .
My character had her right leg crippled by fish monsters and so 5ft of my movement was taken from me . . . (please note that other people were almost eaten whole but they didn't suffer any reprocusions)
And as stuff was getting worse for my character, I put all of my gold into getting a set of split armour to boost my ac to 20 as it was the only tangible thing I could do to boost my tankiness at the time and then my character kind of became stagnent in terms of progression from there . . .
Then he just started throwing enemies at us with incredibly high strength (and resistance to physical attacks) and so my character just got knacked anyway whilst the rest of my party decided to do silly things until I would get knocked out . . .
Then I would just have to walk around with 9 hp cause they wouldn't let my character rest and then we would have to fight things like Flail Snails with annoying AOE attacks . . .
and the group just sort of ended and I kind of took it from my experiance that I could of done better which I think I can now . . .
But yeah, getting knocked out was my thing . . .
So, what I'm hearing from all this is:
1) Your DM focused a lot of damage your way
2) Your DM rarely had you attacked by things via your high AC instead prefering saves which always did half and targeting you with effects you weren't resistant too
3) You got little to no support from your party, even (while being the only front liner) being forced to continue adventuring without so much as a short rest to recover your hp
And despite all that, you did not die.
Your champion sounds like an amazing character, who put up with far more than his share of adversity.
But, if you want advice, it really depends on what you are looking to build.
A Goliath or Hill Dwarf Champion with Defensive Style and Heavy Armor Master is going to be an impressively hard character to kill, especially if you grab durable or tough later on. Same if you multi-class into some barbarian for rage resistance, but then don't try Heavy Armor.
An Archer character is as simple as a high dex and the archery style, eventually I'd grab sharpshooter.
A good mix of defense and offense can be had with sword and board via duelist style.
Big damage can be a two handed weapon in the hands of a half-orc.
Each style gives something up, so what do you want to have. Big AC, Big Hp, Big Damage, or a versatile mix. Champion can do it all, it's just a matter of what kind of build you want.
Also, if you get to high level the extra saves, personal healing, and eventual Regen make you a tough individual to put down.
Each type of build
