Via waiver - usa - guatamala - usa

I am currently living with partner in USA under V.W.P.

If you are "living" with them, you are breaking the law.
If you are "staying" with them, things are just complicated.

What are my chances of being granted a new 90 days?

Between 0 and 100%, inclusive. What does that mean? It means know one knows, as each entry is at the discretion of the official you get upon arrival, so you can't know until then whether you will get back in. The less you conform with the regulations of the VWP (not having onward ticket within 90 days, working, taking up residence, etc.), the less likely you will get back in.

If you are "living" with your partner, expect to be denied.
If you are denied, you will
1) be sent back to Guatemala,
2) have to find your way home (your non-US residence) without transiting the US, and
3) be ineligible for the VWP in the future.

If I hand in my Green departure card when departing from L.A or S.F.O,

Actually, you are required to hand in the I-94 as you are leaving the US, as well as Canada, Mexico and the Caribbean, the areas that are relevant for the VWP. If you tried to return on the original waiver, yo probably would be denied entry.

Don't forget that when you return to the US fro Guatemala, yo must present an onward ticket that goes to somewhere outside the US, Canada, Mexico, or the Caribbean within 90 days.

As #1 says, the only answer is "who knows?" The US State Dept. says:
>How long Must I be Outside the U.S. to get another 90 Days on the Visa Waiver Program?
>There is no specific time limit on how long you have to be outside the U.S. to be able to go back. The point is that the VWP exists for temporary visits to the U.S., and not for enabling persons to more or less live in the country. It is thus not sufficient to just slip over to Bahamas to “charge the passport” for a new set of 90 days. Ultimately your admittance is decided on at the border. A good way to foresee whether you can go on the VWP is to ask yourself “am I here in temporary business or pleasure, or am I just trying to stay in the U.S. for a long consecutive period of time?” Please note that staying with an American boyfriend or girlfriend for a long period of time is not considered tourism.

AND

There is no set limit to the number of times you may travel visa free under the Visa Waiver Program (VWP) during a given period. There is also no minimum period of time you are required to remain outside the U.S. before reapplying for admission. However, if you are a frequent traveler to the United States, you should be able to present the following to the U.S. immigration officer upon arrival:
• evidence of your residence abroad to which you intend returning at the end of your visit
• evidence of funds sufficient for your planned stay in the United States.
If the immigration inspector is not convinced that you are eligible under the VWP, you may be denied entry to the U.S.