BF ex GF went as his plus one to a wedding.

Budget. Sum1 should've warned them!

If she was invited ‘along with him’, she is indeed his Plus One. It might not be as literal as ‘Name +1’ written on the invitation, but she’s still being invited as the cousin’s partner. In those circumstances it would be pretty bizarre to still go on the basis of ‘it’s my right and you can’t stop me’. If that’s how she behaved no wonder he ditched her.

They were a couple when the invitations went out, so it wasn't "bloke plus partner", her name would have been on the invitation.
I doubt she thought it's her right to go no matter what. When couples split, the families don't always ditch the ex partner immediately.
My cousin and his wife split last year, and she has come to several family gatherings since, and been very welcome, we haven't all just turned our backs on her.

Wow he doesen't sound into you at al. So he would rather go with ex, then you, he kept it from you. That is a big betrayal. That would be a dealbreaker for me.

Think it's bad, particularly because he never told you - years ago I left someone because he was much older but we remained good friends for a long time and spent xmas etc together just after I met someone else, but I told him and explained and there was nothing in it - if your bf decided it was best not to tell you it doesn't sound good tbh

She would have received a wedding invitation along with him when they went out, before they split.After they split it would be her choice whether to go or not, she wouldn't be his +1. He could probably have told her he didn't want her to come, but it would be her right to go as she received an invitation, and he couldn't stop her.

If she was invited ‘along with him’, she is indeed his Plus One. It might not be as literal as ‘Name +1’ written on the invitation, but she’s still being invited as the cousin’s partner. In those circumstances it would be pretty bizarre to still go on the basis of ‘it’s my right and you can’t stop me’. If that’s how she behaved no wonder he ditched her.

What’s more likely is that she got on well with the family and the two split on good terms, so went together rather than waste an already booked place.

However, as the OP has concerns that this man is keeping her at arm’s length, I think she would be wise to ask herself if the ex was quite as ‘ex’ as he claimed. Was there ‘crossover’? Or is it still going on and the OP is his unwitting bit on the side?

If you've been having problems since the beginning then you don't have a relationship, you have a chore. The first part should be the absolute best; it's what hooks you and makes you want to be with the person. If it's awful, there's nothing holding you together.

We've been having relationship problems since the beginning

Well that's not really a good sign!

"- it's ALL about him not mentioning it to his new gf."

Yup!

Could he have been seeing you both for a while?

You need to talk to him about it, it’s the only way to put your mind at rest.

And ignore Midnite, you made a small error – it happens – spelling/grammar pedants are insufferable.

It's not about her going - all of the 'they might just be friends', 'they might have wanted to avoid upsetting the bride', 'maybe they were invited separately' is fine - it's ALL about him not mentioning it to his new gf.

I'd be unhappy to find this out and not have been told. In fact, my now-ex told me he was going to visit an ex for dinner, about 9m into our relationship, an ex who had been a total pain, I was fine about it - but later found out he had 1) stayed the night without telling me (I asked specifically afterwards as I was suspicious of not having heard from him all night) and 2) slept in the same bed as her (again, after I knew that he'd stayed I asked and he made a million excuses as to why he had to stay and had to sleep in the same bed) - he protested that they were just friends and that I was insanely jealous and untrusting of him to even bring it up. I wish I had dumped him right then and I like to think I would not tolerate that sort of behaviour these days.

Could it be that she's friends with them, but they couldn't afford to invite another two people (his +1 and her +1)?

Ironic though, that MidniteScribbler can't spell her own first name correctly whilst criticising the OP's grammar.

Not so clever now.

You know deep down what you think and this has just confirmed it to you. There are lots of explanations that might make the situation OK but you've decided not to ask... Because you know what your instinct is telling you to do. Listen to it.

If they were together a fairly long time she may have been close to his family. Also wedding invitations generally go out months in advance so in all likelihood she was invited way before they broke up. Doesn’t mean she had to go though obviously and I’d imagine it would be awkward actually. That is unless they’re still together and you’re unknowingly his side piece?

Either way I would be ending things.

I've been to a wedding with an ex, we were both friends with the couple, and we were still friends, so didn't see anything wrong with it. I did tell my new BF though, and he was cool with it.

I think the fact that he didn't mention it is a bigger concern, and it sounds like your relationship is in a bad way, tbh.

If I was in your shoes, I'd bin him and move on.

OP you sound very, very unreasonable.

You've seen a photo of the two of them together at a wedding. From that you've assumed:

1. she was his plus one
2. he's kept that a seceret from you
3. the bride should have moved the seating around
4. the exgf should have realised she shouldn't attend (no idea why you think that)

Talk to him? You know- communicate!

"The fact that you stalked him"

Looking at photos of someone on the internet is not stalking.

The fact that you stalked him, if you did, and you wont ask him, speaks volumes

We've been having relationship problems since the beginning as I never felt part of his life and this is the nail on the coffin that it's not all in my head

This isn't really a promising thing. You should have ended this ages ago OP.

As for the ex, if they ended things on a decent note and the invite was for the both of them then why shouldn't they go together. A new girlfriend of 3 month couldn't be expected to be invited surely?

Surely if she was invited in her own right they bride would have moved her onto a different table

Her moving onto another table would mean someone from that table moving to the table she came from and not many people go to weddings completely alone so you’d end up having to move a couple of people and in the end you’re basically re-doing your seating plan. A few months before the wedding I wouldn’t be doing that.

I also would finish it. It is too early for relationship issues, and there are now issues of trust. He has not been honest with you. It is not worth wasting your life with someone that has such a lack of judgement that he allowed the wedding situation to happen, and then didn’t tell you.

Trust, respect and honesty are the cornerstones of all good relationships.

Maybe he just wanted to go with someone who knows the difference between "shouldn't of" and "shouldn't have"?
Yes! I could start a whole new thread on how much that particular error irritates me.

We've been having relationship problems since the beginning

That's shit. You haven't even been with him for a year.

So no honeymoon phase whatsoever.

Dump and move on. This has got disaster written all over it.

He hasn’t got over her yet.
You are a rebound relationship :(