"Aがこわい" commonly means "A is scary", "I am scared of A". So 私がこわい is commonly interpreted as "I am scary". For example, 昨日、友達に意地悪をしてしまった。私がこわい.

If you want to say "I am scared", you can say "私はこわい", "私はこわがっている".

However, "Aはこわい" can mean both "A is scary" and "A is scared", so if you clearly want to mean ""A is scared", you can say "Aはこわがっている".

"Aがこわい" commonly means "A is horrible(scary)", "I am scared of A". So 私がこわい is commonly interpreted as "I am horrible(scary)". For example, 昨日、友達に意地悪をしてしまった。私がこわい.

If you want to say "I am scared", you can say "私はこわい", "私はこわがっている".

However, "Aはこわい" can mean both "A is horrible(scary)" and "A is scared", so if you clearly want to mean ""A is scared", you can say "Aはこわがっている".