I agree with setting the boundaries, that's your initial defense mechanism to start with.
But I'm seeing a lot of hostility towards jealousy in the thread, but jealousy is a completely natural emotion. In her case it's very strong obviously, but what that means is that she needs to learn to manage her jealousy, and she might actually need some help. Her behavior looks to me inadvertent and non-malicious, but more one of a lack of maturity on the topic.
Now that you've set the boundaries, you can choose your role in her self-help process. One way is to remain on the sideline and leave her on her own to figure it out, and just continue doing what you're doing. That's the easier way for you.
The other way, is to have compassion, and realize that she needs some help. If you are willing to spend the time and energy for it, then you can assist her by searching into the matter, finding books regarding how to manage your jealousy, seeking professional help like therapy videos and sessions, and guiding her to get through these professional sources.
Judging from you saying that you really like her a lot, I think you might side with the second way.
Your question was:
Is there a good way to start a conversation to solve this with her? I
really like her a lot.
Yes. Here are some steps I recommend to start with:
Step 1 - clearly identify the issue with her first to pass the denial phase
The first step is for her to acknowledge the problem at hand - e.g. that she's very jealous. If she's in denial, then you need to first get her to acknowledge it by using examples of her past behavior, until she realizes something's "off".
Honey, I'm noticing that you have very strong reactions when I'm with other girls, I feel like I'm suffocating sometimes, but I know that deep down inside you trust my judgement and I trust yours. I think it's normal to feel jealousy, but in your case I think you're feeling a little bit too much of it. Would you like to work together to find a solution and learn to manage your jealousy?
Step 2 - She agrees to it, now it's time to help her equip herself with tools to manage her jealousy
You're not a psychologist, and you must understand that you might not be the best person to assist her. However, as her boyfriend, you can definitely help her seek some professional help, therapy, books she can read on that subject, videos.
There are tons of respectable videos by professionals where the subject is discussed, with exercises, perspective, solutions, ideas.
So without being a psychologist yourself, you can still assist her in helping her to equip herself with tools to manage her jealousy.
Step 3 - meet on a regular basis to discuss if there was an improvement
A good practice is to have follow up meetings, and discuss progress. What happened the previous week, month? Does she feels better and more in control? Do you feel better? Are things getting better overall? Take some notes of what you guys discussed. And re-read them next time to see the progression, as being under the influence of emotions can distort things a lot.
Emotions are a fact of life and can't be ignored, but we can learn to manage them through therapy and life perspective. You're not a professional therapist, but you can take on the role of her guide to try and help her learn to manage her own jealousy, so it doesn't annoy you in the future.
She will respect you for the rest of her life for having spent the time to help her understand and manage her jealousy.