Relationship struggles..

So me and my partner have two children 2&4 and are due to get married next year. We have recently just come home from a long weekend away which was an awful weekend away with the little ones. The first day we arrived we stopped off at a pub as it had a lovely garden for the children to run around in. On arrival my partner orders a jug of beer (1pm) I was a little annoyed as it was still early in the day and thought it was a little selfish as we had the children but at the same time we were on holiday and wanted him to enjoy himself. When we left the pub we got the children in the car and left.. as we left the pub I had realised my partner was drink driving! I politely said ‘ oh god let’s swap over as your over the limit’ his response was ‘ I feel fine ‘ my reply back was ‘ it’s not how you feel it’s against the law ‘ he replied back ‘ I feel fine and we’re only going up the road’ this caused me to get angry and shout as he wasn’t listening. I said your putting us at risk you need to stop the car! So we swapped seats and now there’s an atmosphere in the car. We arrived at where we was staying for the weekend and still continued to have an atmosphere. I sat out in the garden with the children whilst my partner continued to drink. He said he was off to the shop and asked if I needed anything.. when he returned he joined me outside and mentioned he had bought a vape (hasn’t smoked / vaped for 5 years) so course I was angry and disappointed and said that’s it we’re over I can’t deal with this anymore. Come the evening I took my daughter to the evening entertainment whilst partner stayed in with our youngest. Whilst I was out with my daughter me and my partner hadn’t messaged or called each other. A while later me and my daughter had arrived back to where we was staying and both my partner and son was asleep. Something was telling me to check his phone.. truthfully I haven’t checked his phone since we first got together but something was telling me to check. So I looked on his phone and saw he had downloaded instagram( we both decided to come off social media at the start of our relationship) I had seen he had been messaging his ex work colleague whilst I took our daughter to the evening entertainment. There was flirty messages from him to her and also he had been messaging other girls that same evening. I had also noticed he’d been messaging her when we first got together and 3 years ago (again we broke up but literally for no longer than 5 days!) I understand on all occasions I have ended the relationship and we’ve been apart the most for 5 days so I don’t really have a leg to stand on but to be message the same ex work college an hour after leaving you at our holiday home is just gut wrenching! Shortly after me coming back he had woken up and realised I had his phone.. I confronted him about what I saw. He was very apologetic and promised his never cheated whilst we’ve been together and that his been quite stressed with his health over the last couple of weeks and not exactly been looking forward to going away with my parents. He has deleted his instagram account and reassured me I have nothing to worry about and it’ll never happen again and it was a huge mistake. He said he had to much to drink and he wasn’t himself whilst he was vaping and sending the messages. He has done nice things like flowers, card and few gifts over the last few days but I can’t help to still feel absolutely broken. Will I be able to trust him again? I feel I’m looking at a different person now. He’d also been in touch with his mum over the weekend (who I’m not exactly close with) so she now knows our private business and only knows one side to the story! I have said I’ll give him the benefit of the doubt but am finding it very difficult! How would you feel in all of this? Would you of stayed or left? Any advice would be much appreciated, thank you for reading x