Sed replace string within quotes

I am trying to replace string within quotes in a file! Here is an example :

<?xml version="1.0" encoding="UTF-8"?>
<Context>
<Manager pathname="" />
<Resource auth="Container" driverClassName="org.postgresql.Driver"   maxActive="20" maxIdle="5" maxWait="10000" name="jdbc/custom" password="CHANGEIT" type="javax.sql.DataSource" url="jdbc:postgresql://10.10.10.10:5432/database" username="MyUser"/>
</Context>

This command works for several 'fields' but it doesn't work with username because of / .

# It's OK
sed 's/\(.*password=\)[^ ]*\( .*\)/\1"NEW_VALUE"\2/' ROOT.xml

# It's NOK
sed 's/\(.*username=\)[^ ]*\( .*\)/\1"NEW_VALUE"\2/' ROOT.xml

How can I do that please ?

Thank you,