You love your partner, but have begun to suspect ADHD might be a factor in your marital struggles. But you also know that your partner is sensitive to perceived criticism, or doesn't believe ADHD could be a factor. That makes bringing up the topic very tricky. Yet...you still need to do it. Because getting a diagnosis of ADHD, and starting to take ADHD into account as you seek to improve your relationship, are critical steps towards success. Ned Hallowell likes to say that ADHD is a 'good news' diagnosis. By that he means that if you have ADHD you have the symptoms. When you get the diagnosis, you can finally start addressing the symptoms.
In any event, if you believe your partner will be sensitive, here are some things to consider trying:
Read an excerpt from The ADHD Effect on Marriage to your partner. Specifically, find a couple in the patterns chapter that closely reflect your relationship. Read them out loud with a 'hey, listen to this...this is uncanny' sort of attitude. Make sure to pick at least one pattern that refletcs on your behavior, not just on your partner's.
Resist thinking it's all about ADHD. Partners often resist hearing about ADHD because they feel their partner is trying to place the blame for relationship issues on them. They can see how angry or resentful or (fill in the blank) you are...so that blame feels unfair and like an attack. Defensiveness will likely follow. An introduction like this might make your partner more interested: "Wow, I've been reading about relationships that closely resemble ours. And I'm seeing a lot of my own issues reflected back on me, as well as issues that you seem to bring to the relationship. It's really interesting and I would love to share some of it with you." Mention ADHD later, after you have your partner's interest.
Take shame into account. Negative tone of voice and unintended critiques will put your partner on the defensive. Be careful to stay in neutral territory (and tone of voice) as you discuss your issues. Talk about yourself and your own feelings, how you reached this idea. Don't talk abut your partner's short comings, and don't diagnose your partner (you're not qualified to do so, most likely.)
Use an ADHD-savvy therapist. Let them bring up the idea of ADHD. It sounds better if it doesn't come from you...and any ADHD-savvy therapist will also know about your own role in your issues, too.
Use a pre-amble. When entering into a difficult emotional topic, it's good to give warning. You might start with something like "I have something I've been thinking about for a while that I really want to discuss with you. I suspect it will be emotionally difficult, but I'm asking if you could try to stay open and hear me out." Then move carefully into talking about YOURSELF and your experiences, while also referencing that you wonder if ADHD might be a factor...and why.