I (26M) broke up with my partner (22F) as I caught her flirting with another person. 3 months after the breakup, I slept with someone else now I’m full of regret.

I broke up with my girlfriend of 2 years, almost 3 months ago as I caught her engaging in a flirtatious snapchat conversation and was lied to about it when I confronted her.
She was on a night out with her friends while I was at home. Her Apple Watch was going off with this guys name I have never heard of before. I instantly got a gut feeling and knew something wasn’t right. Meanwhile I was in bed, awaiting her reply, seeing she was active on Snapchat but I knew she was replying to him as his name constantly popped up as a notification.
She got home at whatever time and I pretended to be asleep.
Once she had fallen asleep I went on her phone. (The first and last time I’ll ever go on a partners phone)
The guys Snapchat was nowhere to be seen on her conversations list. Weird. I tried searching his first name and nothing came up. I then tried guessing his Snapchat username for 5 minutes and somehow got it. His profile had “accept” which means she had deleted him.
So it being very early in the morning I accepted the request and was greeted with a flirtatious conversation which consisted of meeting up or how they never ended up sharing a kiss. She did say to him “don’t you have a girlfriend?” to which he said no.
That was a kick in the guts as not only did she engage in the conversation, there was no mention of her having a boyfriend.
I took photos of the conversation from my own phone.

I confronted her in the morning about it and at first she wasn’t sure who I was talking about, to then her saying he was trying to meet up with her and that her and the girls were shocked as they all thought he had a girlfriend.
I asked if she said anything I wouldn’t like and she said no.

A few days later I confronted her again. She said she forgot what the conversation was and that I had nothing to worry about. I then also noticed the photos were nowhere to be found on my phone. I don’t know if she deleted them off my phone or I accidentally did something to them but they were gone.

I couldn’t deal with how I was feeling and broke up with her. As said above, it’s been about 3 months now.

Since the break up, she has messaged me every 1-2 days wanting to try again and that she’s sorry and has learned from her mistake.
I have put it off and told her no.
I have been trying to better myself physically and mentally.
I signed up at the gym, started running and doing long walks, put in more effort at work, have become closer to my friends and just genuinely felt happy – with a few sad moments of course, and I hadn’t even thought of or had spoken to another female.

But about a week ago I had slept with someone else, nothing to it, just sex after drinking.
It was my best mates girlfriend’s sister. It wasn’t planned, it wasn’t meant to happen, it just did, I didn’t see any issues with it but I was also a whole bottle of Hennessy down. That doesn’t excuse my actions though.

But now I’m full of regret for sleeping with someone else?
And I don’t know what to do.
In a sense it’s made me want to try again with my ex?
I really don’t know how to explain what I’m feeling but I know I need to do something about it.

Now, half of me wants to try again and the other half just wants to continue with how I have been since the break up.

• I’m not the type to sleep around, I’m a lover boy, I would only sleep with someone if I was in a relationship with them or if I was in a serious situationship. I just can’t believe I slipped up and broke one of my personal rules.

• I don’t like starting new relationships.

• I was already so close to her family and friends. It felt so right and so natural, but I couldn’t deal with what had happened.

• I have been cheated on in all 4 relationships I’ve been in. It all starts the same with me finding out something like what has happened, I forgive them and then I get cheated on again.

• The person I slept with is somewhat close within my friend group – being the sister of a close friend’s girlfriend. She was also friends with my ex a very long time ago.

So I guess my questions are;

What would be the right thing to do if I did start trying again with my ex?

Do I tell her I slept with someone else, or do I stay silent as it was 3 months after the breakup? I am probably 75% sure the person I slept with would tell my ex if we did get back together.

Or do I just continue with what I’m currently doing?
I don’t need to rely on someone else to be happy, I can make myself happy, but that comfort and love from someone is priceless.

If you read this far, thank you very much.
And a thank you in advance to any replies.