Do all temptations come from the devil?  

I agree that we are not tempted by just any outward force but by our own evil desires. The devil tries to tempt us but it will always be our choice whether we give in or we hold on to GOD. There are so many things that we desire that tempt us. How to manage not to be tempted? It is just to look to GOD no matter what. Ask for guidance and wisdom. Let the Holy Spirit lead you to how you should react towards temptation.

Temptation comes from an idle soul, one that is not full of God's word. To overcome temptation then, you should fill your thoughts and heart with God's word through reading the Bible, learning its lessons, reciting the powerful verses with faith and living by the Word. At times confronted by temptation, just relate the own situation to Bible stories and live by its lesson. And of course, you should ask the guidance of the Holy Spirit to help you understand things, because oftentimes we tend to confuse things.

Another suggestion is to busy yourself doing productive things so that temptation wouldn't come into mind easily. Well the truth is, temptation is a means to gain merit in heaven so when temptation comes, take it as an opportunity to prove yourself child of God.