In reading your post, I feel like all relationships come with a clock attached. Even going out to "til death to us part" is an ending, right? So could take the bull by the horns and prepare for the best endings possible.
Maybe tell your partner something like:
i am terrified that we might breakup, even though i am happy in the relationship and as far as i can tell you are too. Are you? Am I aware of your wants and needs? Am I meeting them?
I don't want to be treated like I was in previous break ups. Please talk to me -- hopefully it will not be needed. But please talk to me about how you want to be treated and how I want to be treated if we should find ourselves contemplating breaking up. Hopefully we have a conflict resolution method in place we both can do that can help us solve whatever it is but let's say it is unsolveable. And we have to think about breaking up.
I want to to be treated respectfully like _______. What would you want it to be and how do you want to be treated?
Sometimes knowing there's a plan for how to deal with conflict, and a plan for how to deal with breaking up can help soothe fears. Expectations for being treated well in relationship -- regardless of what is actually on the table -- can alleviate the fear of being treated badly. The sharing of vulnerable creates emotional intimacy, which also can help alleviate the fear of being treated badly.
And if everyone is feeling all kooshy in there communicating their wants, needs, and limits... You up the odds for staying together AND you are keeping it real. Both help with relationship confidence.
how do i learn to stop comparing relationships to each other, stop negative/destructive thoughts, etc...?
Since you seem to recognize that you do it... could decide to stop feeding that wolf.
Feed the other one instead, and consistently choose behaviors that feed the other one. In your thought pattern, you talk pattern, you behavior pattern -- YOU choose the wolf you want to feed.
If unable to do it alone or with partner support and you need professional help, get help. Maybe there's a Recovery chapter
near you. Or maybe you want one-on-one counseling. Explore your options there (which BTW, is you choosing and action to feed the white wolf and not the black one.)
thank y'all for reading through this pile o' crap...
You are welcome.
I hope that was a joke. But even so... stop talking down to yourself like that. You are human. You are not crap. You are not a joke. Could strive to talk about yourself EVEN TO YOURSELF in self respecting ways.
These things matter to your well being and long term healths.