Why would you want to assess your own maturity?
Certainly not so that you can just feel bad. Discipleship is a process of learning and growth. No thinking person expects immediate maturity. If you are a new Christian, then being a new Christian is the perfect stage for you. No one condemns a five-year old child for being five!
On the other hand, you would be worried if a fifteen-year old (or a 50-year old!) was behaving like a five-year old.
A personal spiritual check-up can also be inspirational, alerting you to areas that God sees as having growth potential.
Please use these resources wisely. It might be worth considering them along with your spiritual mentor.
Spiritual maturity assessment tools
Spiritual Growth Assessment, v.2 – Peter Cheyne, Followers. Based around 12 biblical values.
Personal Disciplemaking Mini Tool Kit (DiscipleMakers International)
Online Assessment Tools
Assess Yourself – an online questionnaires that provide a score and an explanation
Stonemill Church Spiritual Growth Assessment – seems a little wordy and may be unclear but worth a try.