Hmm. Pageant is an ssh key agent for PuTTY and it's tunnel variant plink.exe. If you haven't ever set up plink.exe, what do you use PuTTY for? Does SourceTree default to using Pageant?
You mentioned you have your publickey in C:\Users\myuser.ssh. Do you also have your private key there? What is the file name of it? If it ends in
.ppk, it is a PuTTY/plink.exe/Pageant key. If it is named
id_rsa and has a corresponding
id_rsa.pub file, it is for use with
ssh.exe, which is the default SSH implementation for Git on Windows.
If you haven't changed the environment variable named GIT_SSH, then Sublime Merge (and command line Git for Windows) should be using
ssh.exe. It is possible that you told the Git for Windows installer to use plink, but that should have set the environmental variable for you.
If you are on Windows 10, you should be able to type "Environment" in the start menu and select "Edit environment variables for your account" to check and see if anything is set.