Before you start addressing any problem, you need to find out what's causing it. Generosity is inherent to your personality, but over-generosity is a sign of a deeper problems.
What causes over-generosity?
Aspects of our adult behaviour are often influenced by our upbringing, so a naturally kind person can start showing over-generosity from childhood.
If your parents went to great lengths to be obliging to others, you tend to follow their example.
You might have heard your mum tell you that you have to be polite and please others at all times once too often, or your dad telling you to share your toys with the others, and being obliging can become ingrained.
This kind of upbringing can cause confusion in a child's identity, because he or she sees sacrifice as a natural law in life. And you can spot it in the playground: there are always over-obliging kids who will give away all their sweets or play the wolf every game.
Fear of authority
If a child is used to obeying others and never questions them for fear of punishment, he or she may struggle to use their own good judgment later on in life, and end up acting as he or she is 'expected' to, by reflex and from fear of disappointing or being 'punished.' This could take many different forms from breaking up with a partner to losing your friends or job.
Need for attention
Subconscious or not, over-generosity is a way of attracting attention and a sign of needing love. People who always foot the bill in restaurants or who are always on call 24/7 for their friends whenever they need them are really showing a need for love and attention.
Uncontrollable leniency with others is the sign of someone with very low self-esteem and who finds it hard to imagine others can think they are worth anything because they have no self-worth themselves.
People with low self-esteem try and remedy it in the wrong way, by spoiling and kowtowing to those around them, letting them get away with anything and not being their true selves. They'll go to any lengths to achieve acceptance, but the results aren't worth it.