I'm thankful you do as much as you do, because people like you are the financial backbone of most Christian organizations (including our own). If it weren't for gifts from people like you, most churches and other organizations simply couldn't continue their work.
You shouldn't feel guilty because you can't give more. God knows your limitations and most of all, He knows your heart. Remember too that He doesn't expect any of us to do everything the needs are too enormous. Instead He gives us the privilege of being partners with others in supporting His work. The Bible says, "God is able to make all grace abound to you, so that in all things at all times, having all that you need, you will abound in every good work" (2 Corinthians 9:8).
Never forget also that God is able to take even the smallest gift and multiply it. Do you remember the boy who offered his lunch to Jesus only five small loaves and two fish? (You can read about him in Matthew 14:13-21.) No one else had brought anything but in Jesus' hands that boy's lunch fed the whole crowd. God can do the same with our gifts.
Most of all, accompany your gifts with your prayers. Churches and Christian organizations need money but most of all they need the prayers of God's people.