how do i learn to trust my partner?

Since I do not know you, or your partner, and the exact circumstances surrounding your discovery of the journal entry, I will offer you some background, that can help you make a determination, as well as some material for your consideration, as to whether to trust, or not trust.

The shortest answer to your question is that before we learn to trust others, we learn to trust our environment, and ourselves, as the earliest developmental stage, in our survival, as individual people.  According to developmental theorists, if we are raised in a setting where parents/caregivers are able to aptly care for us, and feed, and nurture us, then our ability to trust becomes a basic fundamental characteristic of our personality.  If there are failings there, in our family of origin, then trusting will be a very difficult thing to achieve, throughout life.  As we become older, our cultural norms, and the social events that we encounter, will also lend an influence in our continued ability to either enhance trust, and to earn it ourselves.

When we incorporate trustworthiness into our way of being, we can then model that for others in our relationships, and we know deeply what trustworthy behavior is in others, and in ourselves.  Then, we can appraise behavior that is trustworthy, and behavior that is not trustworthy.  Someone lying to us, is not trustworthy behavior, and therefore we should not learn to trust it, because it will lead us to a damaged end, with possible harm.  In the same light, if we compromise the trust of others, by being untrustworthy ourselves, then the relationships will shake, and never be stable enough to merit trust.

Therefore, one conclusion to be drawn from all this, is that if we are raised in a trustworthy and nurturing environment, then we learn to trust, and we also become trustworthy.  This also includes the notion, that we learn at an early age what is not to be trusted: Lies, and attempts to deceive us.  We should never ignore those, and even if we love the person, a lie is still a lie, and a betrayal is still a betrayal, and not to be trusted, because it will lead us to harm.

Your question is: How do I learn to trust my partner?  This is followed by an instance and discovery of something that logically shakes your ability to trust that partner.  The discovery itself, is something that did not seem as though it was authorized by your partner, or was it?  Were you allowed to read your partner's personal journal?

Your partner also offers you a reason for writing what was written, which is at best questionable, because in writing one's feelings, ADHD is mitigated by the conscious deliberate cognitive process of formulating a thought, into a written, grammatically sensible sentence.  So, impulsivity, agitation, and distraction are more easily filtered out of a written statement, especially as it concerns a stream of consciousness that is captured in our innermost thinking, in a personal journal.

So my first offer to you, is that:

You are both doing things that lead to distrust, and therefore may not result in learning to trust one another.  You are seeing your partner's inner thoughts, and making your own conclusions about what you may have discovered in the nuanced intimacy of their journal, and that is not a trustworthy action, which may have led your partner to lie to you.

Your partner is also offering a seeming defense of what was written, which makes little sense, instead of saying something like: "You know this was my inner thought, and yes, I find this person attractive, but it does not mean that I will betray you with this person, any more than I will betray you with all the attractive people that I see in the street, and deal with outside of our home".  If this was said, I would find this statement more trustworthy, because it is more linked to a real genuine thought, and certainly in reality we do not always do, what we may just like, fantasize about, or find attractive.

So, please know, that a thought, or a desire, does not equal an actual plan to do something.  Writing down one's feelings, is a great way of lending a positive channel to a number of risky thoughts, that do not necessarily lead to doing something about them, or planning an action.  This is especially true when we write something in our own personal journal, and do not send it to another person. 

Secondly, I also offer you this for your consideration:

If you wish to trust, you must be trustworthy, in your intimacy with the other.  This is how you learn to trust, by walking in that path, and understanding what it takes to earn someone's trust, because then you can benefit from this walking wisdom with the person that you are being intimate with, and teach that person your brand of trusting.  You can then easily see how they take to that, and how they act, and reciprocate, in order to generate, and earn trust themselves.

Finally, also consider this:

If you are being lied to, and you discover this to be the case, then you should not trust the person who is lying to you, because their attempt at deception, may lead you to eventual harm, in that relationship.

I hope this has been helpful, and can assist you in determining how you can learn to either trust, or distrust.