Does sound like dissociation. 3 responses to managing dissociation:
Building up grounding strategies to help you reconnect to the present time and place. That can start with literally focusing on how your feet feel on the ground to listening to birdsong or traffic noise around you.
Focus on establishing a sense of safety. Perhaps by creating a safe place in your mind that you can visit when feeling panicked to help you settle.
And strategies to bring your anxiety level back down - breathing properly is important (the breathe app might help). Some people have a small soothing or distracting object they might take out with them that they can hold (something small enough to fit into the palm of your hand or keep in a pocket can be good).
Those are the 3 key things to focus on when this happens or you're feeling anxious.
As for not allowing yourself to become too afraid to go outside - combine the above with repetition. You need to go back outside repeatedly and consistently to re-teach your brain that you are safe outside. The more you avoid it the more the fear and anxiety will be reinforced.
Exposure therapy ladders can be useful. Start with a short, easy trip outside (doorstep, end of driveway. End of street, could be your first 3 rungs on ladder, etc). Stay outside while anxiety peaks and fades - because that's what it does. Use your strategies. Come back inside after the anxiety and any dissociation has eased. You need to teach your brain that it's safe, it will pass and you will survive it, so it's important you stay out long enough for anxiety to peak then fade.
Repeat each step a few times until you feel comfortable then move up a rung on the ladder to something a bit more challenging. Repeat, get comfortable, go up a rung until you can go for walks like before.
It's important to do it gradually and repeat each step until you feel comfortable. If you just leap in with the most anxiety provoking situation without gradually building up you're more likely to make things worse. A gradual approach is proven to be one that leads to success.
If you Google CCI resources there are online modules that may help.