How many chances should a guy get?

I've been dating my boyfriend for 3 years now. We went through some tough times in the beginning with him flirting with other girls online. Well, it stopped for almost 2 yrs (as far as I know). He has pretty much been living with me for this time, but we have had problems because he was laid off from his job over a year ago and seems to be very unmotivated so this has caused a little tension. We had a pretty bad argument about a week and a half ago and he went back to his mom's house. He decided that he would move out of my place until he gets a job. Well...we still talked as much as he would talk before so I didn't think anything was wrong. He came to visit me a few times during the week. I guess I sensed something was strange but thought it was just my mind playing tricks on me since we were around each other so much before. I called him one morning before I went to work to ask him about my tire (having problems) and he didn't answer the phone (which wasn't uncommon because he's been sleeping in late since getting laid off). I also sent him a text. He ended up calling me several hrs later and yelled at me and was very rude about me calling so early. I was so upset and he immediately apologized and said he would see me when I got off work because he wanted to stay with me over the weekend. Well, that day he was waiting for me at my place after I got off work. He seemed to be excited to see me and he said he would be right back later after his baseball game with his buddies. He came back and was in a good mood and I was feeling like it was promising. Here's where things went down. He went outside to smoke and his phone went off (it was sitting on my table). He received a text message from a number that said "hi". So, I asked him about it when he came back in and he said he had no clue who that was from. I just had a bad feeling in my stomach...and also have a great photographic memory and went to the bathroom to text the number. Long story short the text was from a girl he had been texting for a week. She happens to be a family friend living out of town. She said he texted her nasty stuff, wanted her to talk dirty, asked her what she was wearing and the worst part for me she forwarded a text where he said "good morning my sweet n beautiful" at 6 am...same morning I called him about my tire. I'm devastated now. He admitted to doing it and of course said "I don't want that stupid little b****" and "I only did that because we were having problems" and "I'm a dumbass and made a mistake" and "I'm sorry", etc.....I don't know what to do. It's so hard for me now because I love him and thought we were going to be together. I just wanted neutral opinions on if I should give him yet another chance.