Resin problem with high temperatures

Hello and congratulations for your site! It’s really helpful. Me and my boyfriend are making jewelries with resin and wood. After we exposed them to the sunlight or in high temperatures such as 30 degrees, resin softened too much. We could scratch the resin but gradually the scratches disappeared when the temperature was low again. We bought another epoxy resin which is suitable for industrial use but after sun exposure the color dissappeared a little bit. What can we do to avoid that problem? Thanks in advance!