Not sure what to do. I (29F) am supposed to get married and I caught him (29M) being shady. Can people change their ways?

I (29F) have been with my partner (29M) for 3.5 years and we are engaged. I thought everything was perfect and headed toward a bright future. Here is when the downward spiral started.

Background about how we got into this mess - A woman he was friends with in highschool is on his snapchat, and because I knew of her self-admitted love for him years ago and I don't love snapchat, I said it was fine to keep her on snapchat as long as he is transparent about their interactions. They don't interact much aside from responding to each other's stories every so often. I knew she should have been on his message history from 30 weeks ago. He showed me something on snapchat and then I saw her name was gone. I asked what happened and why her name wasn't there anymore and he said he didn't know. So I took his phone and searched her name, and he deleted a convo they had a few days prior (she responded to his story - no big deal). The big deal is the lack of transparency and the lies.

The plot thickens. I downloaded his snapchat data to my phone (please don't come for me - I have to look out for myself and I would be okay with him doing the same). Anyways, I found out that in the past year, he has added and deleted about 200 women from snapchat. Most of them are randoms that are not from our city, and some of them are from our city. He deletes them after a few hours or the next day, rarely a few days after.

I confronted him with this information and pretty much said, why are you concerned or preoccupied with other women when you asked me to MARRY you? There was about 4 women from our city that he would add and delete every like, 2-3 months but lots of random users between those times. Like every couple days. They never accepted him it looks like, and there was no convo. But WHY?!

He can't explain himself more than "I fucked up" or "I deleted them because I felt guilty/I just added them by accident, they're just bots". But he is failing to answer the question WHY. Why he had the urge to add these women from our city, repeatedly, and other random users.

So, there are some problems that we have in our relationship, and his best excuse is blaming those problems. As in, when stuff gets hard, it leads him to do that. But I don't understand 'why'. When stuff gets hard, you're not supposed to be preoccupied with other women - you're supposed to look within and work on the relationship, choose your partner every day, etc.

To sum things up, do people change? Will the preoccupation with other women lead to worse things like cheating? The preoccupation with other women when 'things get hard'... like we don't have children yet, things aren't even HARD yet. The fact that he was being shady, lied to me, hid things from me, was preoccupied with other women, deleted a convo, etc. really hurts, even though he didn't 'cheat', I feel betrayed and confused. I would never be concerned with another man. He is saying he will do anything he can to regain my trust and that nothing like this will ever happen again. Do I go forward in our relationship?

TL;DR - Fiance has been shady on snapchat, adding/deleting ~200 women in the past year. No convo history but preoccupation with other women and justifying it 'because some things in our relationship have been hard'. Also deleted a convo and lied about it. Will this lead to worse (cheating) or can people change if they want to put in the work? (He says he does).