In need of some serious advice

I have been with my current boyfriend for almost five years now; he was the one I lost my virginity to at 16 and I saw him as someone whom I wanted to spend forever with. I quickly grew attached not only because of losing my virginity to him but because he was my first ever serious relationship(I've never been with anyone else) so I automatically thought that he was the one. I grew to love him for everything he stood for. Not knowing exactly what I was getting myself into I am one to assume that when you have something great you don't do anything to fuck it up. I wasn't his first however, he's been with multiple girls (he was definitely a man whore) but when I first met him I didn't even really know who he was, we went to the same high school but he was a year older than me so I didn't know too much about him. I also assumed that when you want to be in a relationship with someone you are exclusively for that person.  Well it started off great of course, he treated me how I thought I should be treated up until about two years into our relationship he cheated on me with a girl who was new to our school. It was embarrassing. Walking the hallways getting stared at and talked about...people feeing sorry for me. When I asked him about it of course he lied and when I had proof about it and confronted him he would make excuses. But me being a your teenager who thought she was in love fought for him regardless. Now I can't tell you why I did but I thought you know people make mistakes and I let it go.(yes I know once a cheater always a cheater) but I'm so insecure that I blamed myself. I thought I wasn't good enough and I tried so hard to be good enough for him and just when things were going great again I was wrong. Two years later (2014)...I noticed strange behavior and a girl on his social media that just wouldn't go away. Me going with my gut instinct I would check his phone and this girl was in his messages and liking al of his statuses on fb and pictures and there was a text between them where he asked her how her legs were feeling and I confronted him on it and he goes "oh it's an inside joke we were wrestling" at that moment sadly I was stupid and have this guy the benefit of the doubt not thinking anything...time goes on (a couple months) and she posted a picture of herself on his bed and let me just add this she was only 16 still in high school and he was 19 going on 20 at the time. All in all basically as I was trying to be all I could be for this guy he was fucking around with a teenager while I was at work and while I was crying at home trying to figure out what I was doing wrong. Come to find out in a shitty way(he and I were having a sleepover and when he went to the bathroom I looked through his phone and there were messages from this girl saying something about a pregnancy test and that she was pregnant) my heart sunk. For about a month he was taking her to her appointments. I had no idea. I confronted him and of course he started with the waterworks and shit and me being stupid stayed. I know I'm stupid for that but I had no confidence to leave because he made me so insecure and broke me down so much. He was what I thought was my first love. Might I also add that this girl knew he was with me and she didn't give a fuck. And she got pregnant but she didn't know who the father was because she was fucking around with multiple people. Sadly though I'm currently still with him and he still hasn't even taken a paternity test. Every day I think about it and how he could have a son out there. I see pictures of the baby all the time and he looks so identical to him. I just want to know how does one build up the confidence to break it off with someone? I have been mentally and emotionally preparing myself to be able to break up with him but for some reason idk how to hurt his feelings. Idk how to hurt him at all. I just need advice on how to move on from such a disaster of a relationship.