Don’t know what to do?!

Okay so I’ve recently been on and off with my boyfriend (of a year) I love the boy to bits he does a lot for me and I know he loves me we’ve been through so much in just a year. however when we’ve been off just recently he has been messaging someone that he kissed and she had feelings for him (she told me this) so last night he came round to see me and his phone lit up and it was her on snapchat (little did he know I already seen the chat when he went shop 🙄) so I say to him oh you guys are talking again he says no she randomly popped up and he hasn’t heard from her ( I know this is a lie as I’ve seen they’ve been talking for days planning to meet) I just thought it was a bit weird as he had previously blocked her off everything and so did I go try make things work better. He lied straight to my face. Then after admitted it even though he has the chance to be front with me so many times . Like an hour before this we agreed to take things slow and try work it out because we tend to get full on basically living together . But now that he lied to my face I’m questioning it what should I do

*we hadn’t been speaking for about a week and he messaging her few days later but it’s what’s bothering me the most is that he just lied to me straight up what should I do ?