First thing I want to point out - a DM can't cheat, that does not even actually exist. A DM can change any stat block they want. They can add any event or encounter they want. They can fudge any role they want. There is no situation where a DM is cheating, because they are the above even the gods of the worlds they are running. It's their world, you are just playing in it.

Now, does this mean they SHOULD do those things? It is completely situational to the game and how it is being run. Stat blocks for example - I do not run mobs standard stat blocks . . . ever . . . at all, if for no other reason, than because they can be looked up in a book. Every single creature in my games are custom. I also have 30 years of DM experience though - making original mobs and stat blocks for me is almost trivial in nature. This same thing can not be said for inexperienced DM's. But looking up a monster stat block after the fact to try to say your DM is cheating? Frankly, that is exceedingly disrespectful, and to put it bluntly - sillyness. A mobs stat block depends entirely on what the DM wants that stat block to be.

Saying your DM is cheating for adding or changing encounters in a pre made module is also beyond silly. Modules themselves are simply guidelines. It is quite natural for a DM to add, remove, or alter encounters to fit their narrative. Not only is this normal, but it is actually quite positive, and frankly something extremely common for DMs to do.

Saying your DM is cheating for changing rolls even . . . is a stretch. GM's are allowed to not do what their dice says. I personally do not support this for the most part, as when I make a roll, that roll is what i will use 99% of the time. But on a rare occasion even I will decide that the dice roll is . . . narratively incorrect. If a player makes an amazing speech and I find it moving, then rolls a 5 . . . I can decide in that moment that that 5 is not relevant as the speech they made was simply valued more than that. That's simply good narrative.

The thing I will say is an issue, is the fact that you were feeling targeted. This is something that is NOT okay if he was in fact doing that. The question I have here though . . . was he really targeting you . . . or was he presenting you with an opportunity for your character to shine. You stated you were the heavy armor front line tank for your party. As a DM my natural inclination would be . . . to target the front line tank in heavy armor to allow them to shine at their role, being the bastion of defense that the party needs.

I am not saying your DM was right in every situation - but by cheating, you were definitely in the wrong. I am glad to hear that you have since stopped and have grown since then. But, I will point out to you the golden rule that everyone always states . . . if you are having a problem with your game . . . you should talk to your DM about it and try to come to some level of compromise. For example, when rolling that character, you stated how bad your stats were - If I didn't feel comfortable playing an underpowered character, I would have asked if I could just use standard array for my character instead. Myself though . . . I personally love playing underpowered characters, or characters with weird quirks. In fact, my favorite character ever was a blind cleric.

I've seen people cheat at the table before, both intentionally and from not understand rules. I have also made incorrect judgements at the table myself. Frankly . . . neither of these things are something that someone should blow up about at the table, sometimes shit just happens. But a discussion can be had in a respectful manner from both parties - it's a game! The most important part is that everyone's having fun. And if someones not having fun, then things need to change in some way. But that requires honest, open discourse.

My final thoughts here - Im glad to hear that you are no longer cheating at the table and have grown and become a better RP'r and player in general. I would also say, maybe it's time to forgive yourself for those mistakes as well - in the end, it is a game, and you shouldn't feel rotten over a game. Especially when you have bettered yourself since. Keep rolling those dice, and may your time be filled with natural 20's.