When they ask you why, can you say that you just dont feel ready yet.. without going into detail? Keep saying, "i just dont feel ready yet" no matter the question they ask or what they say. You have a right to feel this way. Can you say, you dont know exactly why you feel this way but you just dont feel ready yet? Maybe you can say you feel overwhelmed and want to do it for the right reasons, not just because everyone else thinks you should or grandma is planning to come.
Maybe say you hate to disappoint grandma and feel even worse now that they told her.
Redirect the guilt on them for making you feel guilty. Keep saying things like, I feel so bad for not getting baptized right away but I just dont feel ready yet. I feel so bad for disappointing grandma but I just dont feel ready yet. I feel like I am disappointing everyone and all of this is confusing you. Repeat this theme to any that question you.
Maybe tell them you prayed to Jah and hope he understands what you are going through.
Hopefully it will sink in to them or the elders.