First distance yourself!
Some people will eternally think that their glass is half empty rather than half full. It doesn't matter how much you talk to them, how much you try and help them certain people just enjoy wallowing! Remember they have to want to help themselves and unless they take the first step to recognise what's missing in their lives there is nothing you can do.
And when you do have to interact with them, imagine yourself surrounded by a bright light that's bouncing back any negativity that they send your way!
Offer positive solutions and if they refuse to take this on board tell them in the nicest possible way that there's not much more you can say and lets change the subject you're boring me now!!!!!!!
It helps to ask yourself, "Is this my problem?" This tends to really clarify and focus your attention. If it's not your problem, discard it, square your shoulders and move on to the things YOU need to deal with. In not taking on other people's problems, complaints and issues, you'll free yourself to deal with your own "stuff."
Focus on the good things in your life. and ignore pestimistics in general. if thier going to be so down all the time, how are you going to enjoy life if its gonna affect you?
First of all, congratulations for being in touch with your optimistic and giving nature - well done! The key is learning how to live with it.
When we are stressed or down, we lack energy and revert to our learnt behaviours (negativity) to provide us a temporary recharge; taking energy from our target. However, we can halt this behaviour:
First and foremost, before interacting with the negative person, ensure that you want to part of the negative persons life; when we truly want something, we become energised. Then the mere act of greeting the person with love and respect benefits both parties: the love and respect we give immediately recharges them and instantly re-energises us. A win-win situation. Once we've started the interaction on a positive basis, the positive energies will feed the relationship.
Give with energy and be re-energised in return.
First of all, assess whether or not you are gaining by having this person in your life. If they are constantly draining you emotionally and not giving anything positive to your life, then it could be that they are using you for your kind nature. Tell them kindly that helping them out is becoming draining for you and that they need to give you a break or to help themselves by developing a more optimistic outlook.
However if you really can't exclude them then allow them a limited amount of attention. If they call for example, allow then a maximum of 15mins conversation and then make an excuse that you have to go. If they insist on meeting with you, allow them only two hours per week. Eventually they will get the message and start to help themselves, or at least, give you a well deserved break.