i can empathise, I’ve felt like this (new relationship after my 20 year relationship broke down)
I did the checking social media, needing constant reassurance, worrying and catastrophising if he didn’t respond to messages quick enough, if he sounded a bit ‘off’ - tired etc but I would turn that personal and worry I’d done something wrong, that he was fed up of me… he honestly was nothing but lovely, reassuring and patient but as you say, the relief from reassurance would be short lived.
best thing I did was to find my own hobbies, limit my social media - I turned off all online visibility so I couldn’t check when he was last online/active and then assume he was ignoring me.
I’ve also reassured myself regularly that, if the relationship ends, if something happens, I would get over it. Yes I’d be devastated and hurt, but I’ve worked on building confidence that it would not be the end of the world because I have things outside of my relationship to be content with - I have family, friends, hobbies, and I actually learned to enjoy being comfortable in my own company
have you tried some mindfulness courses, they may have strategies for calming yourself before you spiral. If I feel myself about to wobble, I start talking to myself logically and reassuring myself rather than free falling into a panic and lashing out
essentially - this might be the totally wrong way but it’s worked for me - I’ve started thinking of the relationship as something that compliments my life rather than it BEING my life
good luck, I hope you find a way to be more settled and confident 💜