Sincerest Apology.

No digression.
Especially to this person

I'm sorry I bailed out on you. I know that feeling. I won't give any excuses. You may ignore, despise or even bail out on me too whenever you want. I'm sorry I'm such a pathetic friend of yours. I'm sorry I let you and everyone else down. I'm sorry I wasn't there when you needed me. I'm Very Sorry.

I'm not giving any excuses, even though they're the valid ones.
All the best for your IELTS.