<human>: Just as I had decided i was happy with the fact I was supposed to be forever alone I started talking to (lets call him )Peter. We hit it off straight away and didnt stop talking since then. I literally felt like id known him forever. 

After a while it came about that he had recently split from a long term relationship  and they had had a child together.
A while into the relationship after he had told me he loved me and I felt the same way too, although held back a bit. He ended up sleeping with his ex. He told me a few days later and I forgave him. I think it hadnt really hit me.

4 years in and I still feel shit about what happened. The fact the person who he cheated on me with is still in his life (and not only has slagged me off for no reason many a time as well as just being as awkward as possible) I have to see or hear about her at least once a week. Sometimes even for pointless things that don't even really have anything to do with the child.

This on top of other things like the fact they contact on snapchat which i suppose its for photos of the child but its kind of pointless seeing as you cant keep the pictures but hey ho maybe thats just me being bitter.

I thought I could move on but i'm not sure I can. I have invested so much time and effort into this relationship that I am so torn I don't know what to do. We could be so good together but I dont know if things from the past will ever really go away and I dont think i could feel periodically crap, whenever reminded about it forever.

I suppose im sort of looking to see if anyone has had any similar experience and what did you do? The idea of starting again is scary and the idea of making the wrong decision scares me also.

Does every relationship have problems like these to deal with? I don't know how I'm supposed to decide if i should leave or not? And i'm not sure if i'm making myself feel so bad by not being able to let go?

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: Am confused after a past infidelity and not sure if i am making myself unhappy and should let go and possibly have a good thing or leave and start again.