Am I overreacting?

[sinninginacorner]

Going to make this as short as possible because I'm in a rush to get out but here goes. Me and BF (of 11 months) met at work, I left in May at the same time as an other female colleague. A few weeks later she messages BF and they start chatting regularly, I have no problem with this and BF is happy for me to see their texts. In august they went on a night out (along with BF's male best friend) and in the days afterwards BF becomes very protective of his phone, not even leaving it around while he goes to the toilet. I confront him which leads to a huge argument and him admitting our ex-colleague has said she likes him. He promises nothing has happened, I have a few days to myself and decide to give him a chance, he promises he won't talk to her again and deletes off all forms of social media he has (so I think). Everything has been fine since and I have had no suspicions.
Fast forward to today and I get a notification from Instagram, which I don't really use but have an account for, asking me to follow my BF. I was bored so clicked onto his profile and looked through his photos (there were only 7). On one from about 2 weeks after the argument about ex-colleague I see she has liked the picture, and BF has commented saying 'miss talking to you hun'. Am I right to be fuming or am I overreacting? I have managed to stop myself from texting/calling BF so far as he is at work but he finishes soon and I need to go to his flat at some point tonight to get a shirt I need for work tomorrow, but I just don't know what to do, I feel so hurt inside because he knows how I feel about this colleague (there's a bit of back story from when we worked together) and I just feel like he has betrayed me. However another part of me tells me I'm overreacting? So Perspective please?

Not over reacting. Not at all. Why would he miss her? Why would he speak to her again after all that happened? What a wanker. I think you'll need to rethink this relationship to be honest

Ditch and move on. Couldn't be bothered with someone being shifty/lining up other girls. Life's too short.

Nope. He stepped over the line, said he cut contact and lied. No trust left.

Oh god, I don't know whether I would have preferred to hear you all say I was overreacting . I really thought he was the one, he's helped me through so much since we got together and until now thought I was the happiest I had ever been

Oh heck. This sounds like trouble.
I would talk to him, tell him you are unhappy and uncomfortable about him being in contact with her.
Ask him what he intends to do about it.
Then have a good think about his response and make a decision.
( I might take a break for a few weeks, then see how we felt about each other)

Just got off the phone with him. He said he could understand why I was upset (well why do it then??) but didn't really say much else. I told him I needed a few days to myself and hung up. He's now texting me like crazy but I can't bring myself to look at them.

The key question is- can you trust him?

It's been 11 months, ditch him now, you'll never trust him again and probably rightly so.

I just can't bear the thought of never seeing him again.

I know that sounds strong but ive never felt this way about anyone before, I feel sick

I know it's difficult, but give it a few days of no contact and you'll start to feel better. You deserve much more than someone who lies to your face and flirts with other girls. Also than someone who ends his sentences with "hun"

Short term pain - long term gain. It is horrible splitting up with someone you really thought was going to be the one, but you will gain self-respect and the knowledge that you can survive this and anything else life throws at you.