Adapted from a recent online discussion.
Question: I've been with my girlfriend for a year. This is my first serious romantic relationship, and I love her very much.
In the beginning, I did a terrible job of managing the fact that she had been in relationships with other people. I was jealous and it showed. I thought I had it under control when an ex-girlfriend of hers moved back into town. During a low moment, I went through my girlfriend's phone and found out that she's not sure whether her feelings for her ex are strictly platonic, and that she lied about spending time with her.
We talked it through (I confessed my snooping) and she shared that she had been wondering but there was nothing between them but friendship. Since then, her friendship with her ex has been a source of stress for us. I asked for honesty about the time she spends with her, and that worked for a bit, but after I got upset because she spent a long evening with her, she went back to seeing her occasionally without telling me.
I have called her out on it a couple of times. She states that when she tells me they're hanging out it's stressful for her. I'd rather she didn't spend much time with this ex, but if she does, then I feel she definitely shouldn't omit that information. I don't know how to resolve this.
Answer: Normally the answer when someone is sneaking around on you is to pull the plug. The only reason I'm not advising that here (yet) is that you might each be half of this problem.
So, here's what you both need to do, even though I'm well aware I have only your attention:
She: stop hiding.
You: stop getting upset.
If you can't stop getting upset, then at least stop showing it until you can get it under control - or break up. Those are your two choices, either trust her knowing it can come back to bite you, or don't trust her and recognize that's game over.
Why? Because you can't give yourselves to each other fully if you're always worried someone will lure her away. You need to believe that her staying with you is her choosing you - and that if she chooses someone else, you'll eventually find out.
Part 2 of this is accepting this might happen, in any relationship, and you will survive it and heal and someday love again. Again - if you can't do this, then you can't be in a committed relationship, either with the person who burned you, if that's the issue, or with anyone, if the fallout from being burned is that you can't trust anyone (yet). As long as you've set yourself up to believe the Worst Thing That Could Ever Happen is for your lover to find someone else, you will always expend a good deal of energy worrying. That will not only hamper your enjoyment of love, but also limit its depth. Repeat to yourself as needed: Most relationships end. And the heart . . ..
Sigh. I realize I quote Woody Allen at my peril, but this Hannah and Her Sisters line says it: "The heart is a very, very resilient little muscle. It really is."