:45 Not sure how I feel about approaching a grounded Peach with a float fair. I think the grounded Peach is stronger in this case, as she has options to stuff this (nair up into you, dash attacking under you as he did, etc). I think a defensive fair here would've been better for controlling that area. Especially since you had center stage.
:50 He pulls a turnip, jumps, and you shield. Not good. Shielding turnips in the ditto limits your options and let's her approach you for free. Mobility is a much better defense against turnips, so maybe dashing away from it, or trying to catch it yourself was better. If you're going to shield, don't stay in for as long as you did.
:57 Not sure what you wanted when you jumped, but in this position, a better defense would've been to stay grounded. Jumping when Peach is around that distance to you isn't good.
1:11 Again, you approach a grounded Peach and get punished.
1:19 An instance where the rule can be broken. He saw you pulling a turnip and hit you. Going for something as laggy as a turnip pull when Peach is close to you (whether on the ground or in the air) is a no-no.
As the first stock ends, it seems like another issue you're having is coming down in the ditto. I'm not sure how you should solve this exactly, but you need to be more aware of where he is looking to hit you as you come down. And stop trying to attack him as you come down so much. (3:04 is an example)
1:37 Same thing again.
2:10 As he faded back, you shouldn't have gone forward with that fair.
2:49 You know better.
When you're a stock down in the ditto, the other Peach is going to love trading. You have to avoid that at all costs. I noticed around here that you were netting trades. Being this far behind, you shouldn't move so far into her when you attack (like 2:52)
3:14 Do you see? Two issues compounded here: You went in for the fair when he faded back (you had an advantage in float timer, so you could've just waited a bit), and when you missed the fair you instantly shielded. Shielding when Peach is above you...not good.
3:21 I'm not sure why you jumped after he landed. You could've dashdance grabbed the fair, or downsmashed it, or done other things besides jumping vs a grounded Peach (even if he was in landing lag). And then you proceed to sit in shield and then you roll and get downsmashed.
3:35 He's just waiting for you to swing at this point, as you've represented so far this match that you're willing to do so before he does, and then he hits you.
4:31 Not sure why you stalled after the turnip. You could've faired him. Then you faded into him after regrabbing the turnip...why? It seems like a weird decision.
4:45 More shielding.
Edgeguarding in the ditto is all about funneling Peach to the ledge, then punishing what she gets up with. A well spaced/timed float fair will punish pretty much anything she tries to do from the ledge.
4:51 And more. It let him restrict your options and got a panic response from you, which he then punished
6:01 There are two instances here where you aerial in and then shield right after, during your lead, and get punished for both
6:31 Attacking grounded Peach again.
So, I'm not sure how to wrap this up...I kind of wrote a lot...but basically, the ditto is just about good decision making, as Mafia said. Bad decisions add up and put momentum in the hand of the opponent. You don't seem comfortable when you're putting moves out, and shield after a lot of them because you recognize that the spacing is bad, or that you're open for punish. However, shielding, in a lot of cases, makes you punishable in a place where you wouldn't be had you moved instead. You've got to stay mobile.
The thing with aggression in this matchu is that you want to control the pace and stay in her face so she can't set up float, pull turnips, and so that you kee control of center stage. Peach loves the center and in the ditto, keeping it is a really good way to win. However, you often approached when Ben had advantage over you (like when he was on the ground and you were above him, or when you were both floating at an even height and you swung first). It's important to know that sometimes you don't need to chase them, but to just control your space.
Ultimately, it's a matchup that you have to play often to get comfortable in, but I hope this helps in giving some idea on why you got hit (I basically just looked at when you got hit and why).