I've been practicing non-monogamy since I first started dating (about 10 years ago), have had one long-term relationship and some shorter ones in that time. I've never really struggled emotionally, above what I would call "just feeling things": moments of jealousy, insecurity, etc. that pass within a day or so, or dissolve when I talk about them and get my needs met.
Now for the past year I've been in a new relationship which I would objectively describe as the healthiest one I've ever had - we're both communicating really openly, asking for what we need, and being considerate towards each other. Still, for some reason, I experience anxiety and insecurity to a degree I didn't think possible. I sometimes feel completely lost and don't even know what I feel, let alone what I would need to calm down.
The thought of him desiring another person or sleeping with them makes me want to throw up, scream, run away, hide and never see him again, all at the same time. I also often feel terrible about myself, like I'm doing something wrong, or that I should be different, should be more confident or should know what to do to resolve this.
That I'm putting too much strain on the relationship and my partner by spiralling for so long without him being able to help or support me in any way. I've never felt this threatened and scared, and have never felt this type of self-hate over a relationship. I don't know why I feel this way and I don't know what would help me.
You’re experiencing anxiety because, as you have described, you’re in the healthiest relationship you’ve ever had — ergo you have a hell of a lot more to lose this time around.
And to make matters worse, you’re expecting yourself to be absolutely fine, which isn’t really fair or realistic on yourself. What if you gave yourself permission to have feelings? I assume your partner is also in on your up and down feelings. I am not sure who has said it is putting a strain on the relationship — has he said this or is it you? A lot of times I feel like people who are struggling with emotions and feeling guilty for their partner’s helping them struggle to believe when a partner says they are okay.
Giving yourself permission to feel your feelings will help a lot with dealing with them day to day, but it might also help for you to tell your partner honestly about how you’re feeling and also trusting that you’ll get better and also that he will be okay with supporting you and be able to put down his own boundaries if he needs a little bit of a break. Also, diversifying your support network so you don’t just have him to help you.
Seeing a therapist might help you with strategies on how you work through some of your anxieties on your own, but overall accepting that you have something that you’re afraid to lose might help. In my 101 and 102 articles, I also talk about releasing yourself from the responsibility to keep someone which may help you counteract this fear.
Give yourself permission to be a little messy. You don’t have to be perfect. I hope this helps and good luck.