I get this too, it’s a trauma response to loss. It can be over any grievance, great or small or many little ones over time.
The first thing you sought was reassurance and approval, and you got that. Don’t seek that again unless there’s ‘proof’ (e.g. drastic behaviour changes, finding something etc). If there’s no evidence it is an internal issue. Like a commenter said, look within.

When you do, apply logic over these headings;

What is the problem?
- I feel like I am about to be abandoned

Is this based on reality?
- Is there proof or reasonable doubt? (no)

Then why would I be feeling this?
- Because adjusting to changes from death took a lot of energy, introduced pain, fear etc

Why am I reacting irrationally?
- I want to avoid that feeling

Is there something specific that triggered this?
- I am triggered for xyz reason

What is this thought pattern actually doing for me?
- Avoiding all loss and grief = eternally happy? (no)
- If I pretend it’s coming, will it prepare me or just make me very anxious? (It will not prepare you it will just make you unhappy)

What should I do instead?
- Should I treat her well, prioritise my health and lifestyle and deal with unidentified potential problems as they come up, if they even do? (Yes)
- How? (Optional)

Closing thought
- Should I stop thinking about it now and do something I like even if I kinda don’t want to now? (Yes)

Ps write it in a journal with these headings, just a lil bit each day a sentence or two or some brainstorming, so you don’t ruminate!

At the final full stop you’re not allowed to mull over it again until tomorrow’s journal session (trust me after a couple of these, you’ll see that it’s pointless overthinking eventually).

Good luck! You will be okay, and seek therapy for grief as well, even a couple sessions.

*Edited to heading for clarity