Merge back MyBranch to Branch1 and then Branch1 back to trunk, or reintegrate MyBranch to Branch1 and then Branch1 to trunk. I'm not a fan of --reintegrate, I prefer manual just merge the right revisions.
----(1)-------------------------------------- trunk \
\(2)_____(3)______________ Branch1
\
\(4)__________(5) MyBranch
Or, if you want go the hard way: merge from Branch1 to trunk the revisions from Branch1 creation to the Branch1's revision from where you created MyBranch, and then merge MyBranch to trunk.
If you know that there's no changes between (4) and (5) that depends in changes between (2) and (3) you can just merge MyBranch directly to trunk.