Welcome ALLi
'hanging over our head has been her claiming she can't trust me, yet we were happy'
I think that part of your answer may lie here.
You thought that you were happy, but you had something 'hanging over' you ~ and that doesn't allow for complete happiness.
You say that the messages to your ex were old ones, but how old?
How long were you secretly contacting her, after starting a relationship with 'the love of your life'?
If this girl
is 'the love of your life', why did you feel the need to secretly contact you ex?
Because of this secret, she feels that she cannot trust you.
That is why she keeps questioning you, because she doesn't know if she can trust you to tell her the truth.
When you kept contacting the other girl you were somehow breaking this girl's trust.
I'm not saying that you don't have the right to friends ~ even female friends ~ but when one's boyfriend is having a secret text relationship with an ex-girlfriend, against her explicit wishes, this is going to upset her greatly.
Why do you think she had the feeling that something was going on?
Have you done anything, recently, to deserve this reaction?
Are you still in touch with the other girl?
Have you been completely truthful about her?
Have you reassured this girl about your feelings for your ex and for her?
Why did you walk out when she said that your relationship would be over if you did?
You said it was to prove a point. What point?
Did you prove a point?
Was it that you are fed up with being questioned over your ex?
If so, do you feel that you have answered her fully about this, and reassured her completely?
If you have explained and reassured, then she may not believe you, or she may not be certain that she can believe you, or maybe she was just looking for any excuse to break away and have some freedom. The talk of marriage may have scared her.
Why might thoughts of marriage frighten her and make her feel trapped?
Could it be that she may feel that you are too young?
There may be a number of inter-related reasons why she behaved as she did, sleeping with the other man.
It might have been on the rebound, or to repay you for breaking her trust, or because she started to feel trapped by the idea of marriage & wanted to break the bonds for a while.
If you love her so much, and she claims to love you, there may yet be hope.
Can you forgive each other for whatever it was that caused this crisis?
You say that you are willing to put all this behind you, but, apparently, she thinks that you would use what she did against her.
What is the real truth?
You may need help from a relationship counsellor to sort things out, learn from your mistakes, forgive and start afresh.
Good luck!
