Sometimes we convince ourselves that if we are not able to do it then no one will be able to. This may be true in some cases, but in most cases there are other people that could step in and do what you have been asked to do, however they may be overlooked or even put to the side because you are always available. I believe there are times when we need to say no so that we can give everything we have to our current projects and also allow others step to the forefront and be able to use their gifts as well.
Keep in mind that we are saved by grace, not our works. Ephesians 2:8-10 states this very clearly, "For it is by grace you have been saved, through faith — and this not from yourselves, it is the gift of God — not by works, so that no one can boast. For we are God’s workmanship, created in Christ Jesus to do good works, which God prepared in advance for us to do." If your motivation is to earn your salvation then your motivation is not a healthy one and you must pray that God would give you a healthy motivation. We should be motivated by our love for God and our love for people, this is what motivated Jesus in his ministry. Throughout the New Testament we see a pattern, Jesus was moved by compassion, assessed the need, then filled it. A great way to determine if your motivation is healthy or not is to ask yourself for whose glory are you doing all of this work?
Lastly, if you do not have time to be one on one with God every day then you have said yes to too much. God is much more interested in where you are at with Him than how many ministries you are a part of. God, instead of our works, should be the center of our lives. If we lose sight of God, we lose sight of why we are involved with these ministries in the first place.
If you are feeling overwhelmed right now, take an afternoon, and spend it talking to God, reading the Bible and worshipping Him. I am certain you will return refreshed and invigorated.